This adds support for so called "layout panels" which are panels that are created as part
of `uiLayout`. The goal is to make it easier to have expandable sections as part of a UI.
The initial use case for this are panels in the geometry nodes modifier. This patch provides
a better solution compared to what was attempted in #108565.
### Problems with Existing Approaches
Currently, there are two ways to create these expandable sections:
* One can define a new `Panel` type for each expandable section and use the `parent_id`
to make this a subpanel of another panel. This has a few problems:
* `uiLayout` drawing code is more scattered, because one can't just use a single function
that creates the layout for an entire panel including its subpanels.
* It does not work so well for dynamic amounts of panels (e.g. like what we need for
the geometry nodes modifier to organize the inputs).
* Typically, Blender uses a immediate-ui approach, but subpanels break that currently
and need extra handling.
* The order of panels is not very explicit.
* One can't interleave subpanels and other ui elements, subpanels always come at the
end of the parent panel.
* Custom solution using existing `uiLayout`. This is done in the material properties. It
also has a few problems:
* Custom solutions tend to work slightly different in different places. So the UI is less unified.
* Can't drag open/close multiple panels.
* The background color for subpanels does not change.
### Solution
A possible solution to all of these problems is to add support for panels to `uiLayout` directly:
```cpp
/* Add elements before subpanel. */
if (uiLayout *panel_layout = uiLayoutPanel(layout, ...)) {
/* Add elements in subpanel, but only of the panel is open. */
}
/* Add elements after subpanel. */
```
Integrating subpanels with `uiLayout` has some benefits:
* Subpanels are treated like any other sub-layout and don't have unnecessary boilerplate.
* It becomes trivial to have a dynamic number of potentially nested subpanels.
* Resolves all mentioned problems of custom subpanel solutions.
### Open/Close State
The most tricky thing is to decide where to store the open/close state. Ideally, it should
be stored in the `region` because then the same layout panel can be opened and closed
in every region independently. Unfortunately, storing the state in the region is fairly
complex in some cases.
For example, for modifier subpanels the region would have to store an open/close state
for each panel in each modifier in each object. So a map with
`object pointer + modifier id + panel id` as key would be required. Obviously, this map
could become quite big. Also storing that many ID pointers in UI data is not great and
we don't even have stable modifier ids yet. There also isn't an obvious way for how to
clear unused elements from the map which could become necessary when it becomes big.
In practice, it's rare that the same modifier list is shown in two editors. So the benefit of
storing the open/close state in the region is negligible. Therefor, a much simpler solution
is possible: the open/close state can be stored in the modifier directly. This is actually
how it was implemented before already (see `ui_expand_flag`).
The implementation of layout panels in this patch is *agnostic* to how the open/close
state is stored exactly, as long as it can be referenced as a boolean rna property. This
allows us to store the state in the modifier directly but also allows us to store the state
in the region for other layout panels in the future. We could consider adding an API that
makes it easy to store the state in the region for cases where the key is simpler.
For example: `uiLayoutPanel(layout, TIP_("Mesh Settings"), PanelRegionKey("mesh_settings"))`.
### Python API (not included)
Adding a Python API is fairly straight forward. However, it is **not** included in this patch
so that we can mature the internal API a bit more if necessary, before addon developers
start to depend on it. It would probably work like so:
```python
if panel := layout.panel("Mesh Settings", ...):
# Add layout elements in the panel if it's open.
```
Pull Request: https://projects.blender.org/blender/blender/pulls/113584
f824476bd5