Weightpaint Gradient tool: expose falloff to the UI

By default, we'll always get a falloff like this from the tool:
{F10559413}

But in the context of using vertexgroups in modifiers/modeling, a choice
on how the gradient falloff of the Weightpaint Gradient tool is shaped
would be desirable:
"real" linear:
{F10559416}
Custom:
{F10559421}
{F10559428}

The way the Weightpaint gradient tool works is a bit outside the usual
tools that use brushes [even though it creates a brush on the fly in
`WPGradient_userData`].
However, it does not have an entry in `eBrushWeightPaintTool` and adding
one there does not play nice for the same reasons (not "really" being
integrated in the brush-based tools).

So in order to expose the brush curve in the UI, we would have to do one
of the following:
- [1] try to use `VIEW3D_PT_tools_brush_falloff`, for this to work:
-- make all kinds of exception in python super classes [`FalloffPanel`,
`BrushPanel`, `UnifiedPaintPanel`, ... -- including making real entries
in `eBrushWeightPaintTool`] to get a proper tool mode and...
-- .. to also make sure Falloff Shape and Front-Face Falloff are not
available [which the tool seems to just not support in its current form]
- [2] just have a simple, contained panel for this tool alone

This patch implements [2] and adds it as part of the ToolDef (could also
be done in `VIEW3D_HT_tool_header`, but again, I think this is nice to
keep separate from the usual tools)
{F10559482}
{F10559485}

Testfile:
{F10559442}

Fixes T91636

Maniphest Tasks: T91636

Differential Revision: https://developer.blender.org/D12614

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1691,6 +1691,7 @@ class _defs_weight_paint:
 
             props = tool.operator_properties("paint.weight_gradient")
             layout.prop(props, "type", expand=True)
+            layout.popover("VIEW3D_PT_tools_weight_gradient")
 
         return dict(
             idname="builtin.gradient",