UI: Brush Settings overhaul

This makes a number of changes to the tool settings brush UI:

  - All brush-related controls are now grouped together, so you can see which items are brush settings are which are not. Previously it was all jumbled together.
  - The brush picker is in a separate panel, so that you can switch brushes without worrying about the settings, or vice versa.
  - Custom Icon settings moved from the Display settings(now known as Cursor) to the Brushes panel.
  - UnifiedPaintSettings panels are removed and the contained options are now next to their relevant setting with a globe icon toggle. This is not displayed in the header.
  - 2D Falloff and Absolute Jitter toggles were changed into enums, to make it clearer what happens when they are on or off.
  - Adjust Strength for Spacing option was in the Options panel in some modes, but in the Stroke panel in others. It is now always under Stroke.
  - Display (now Cursor) panel was reorganized, settings renamed.
  - 2-option enums are annoying as a drop-down menu, so they are now drawn with expand=True.
  - Smooth Stroke and Stabilizer options in grease pencil and other paint modes are now both called "Stabilize Stroke", for consistency and clarity.
  - De-duplicated some drawing code between various painting modes' brush options. I tried to keep de-duplication reasonable and easy to follow.
  - A few more tweaks - see D5928 for the extensive list.

Most of the patch is written by Demeter Dzadik, with some additions by myself

Differential Revision: https://developer.blender.org/D5928
Reviewers: Pablo Dobarro, Bastien Montagne, Matias Mendiola

release/scripts/startup/bl_ui/space_toolsystem_common.py

@@ -378,7 +378,7 @@ class ToolSelectPanelHelper:
 
     @staticmethod
     def _tool_active_from_context(context, space_type, mode=None, create=False):
-        if space_type == 'VIEW_3D':
+        if space_type in ('VIEW_3D', 'PROPERTIES'):
             if mode is None:
                 mode = context.mode
             tool = context.workspace.tools.from_space_view3d_mode(mode, create=create)