For blender/blender!141292, we need a simple icon and text button that
executes a callback on click. The size should be determined based on the
text and icon, as it's typically done with the `UI_interface_layout.hh`
API. Trying to do this with `uiDefIconTextBut()` is annoyingly
complicated, mostly because we'd have to duplicate size calculations.
Instead expand the `UI_interface_layout.hh` API to cover this.
* Allow passing a button type to `uiItemL_()` and rename it to
`uiItem_simple()`. The code in this function is commonly useful, not
just for label buttons. It's a convenient way to add a single button
with normal size calculations. The function stays internal to
`interface_layout.cc`.
* Add `uiItemBut()` as public API function to call this and assign an
on-click callback.
No user visible changes expected.
Pull Request: https://projects.blender.org/blender/blender/pulls/141900
2658754324