Replace 'over-allocation' by a standard `blender::Vector` storage for
the `wmGizmoProperty` target properties of the gizmo. This simplifies code somewhat, and
makes the code ready to manage non-trivial types of data.
Explicit default values are also given to all data of `wmGizmoProperty`,
as it appears that the default constructor will not nullify values in
the (trivial) embedded `custom_func` struct.
NOTE: There is much more that could be done in the wmGizmo area to make
it use modern C++ features. The scope of this refactor is strictly
limited to allowing C++ construction/destruction of the wmGizmoProperty
data, as it embeds a PointerRNA pointer that will soon become
a non-trivial C++ struct.
Pull Request: https://projects.blender.org/blender/blender/pulls/128763
60325a7f8c