This patch allows new GizmoGroup classes to support tool fallback keymap. With this patch, when new gizmo groups add `'TOOL_FALLBACK_KEYMAP'` to its `bl_options`, the fallback tools are added to the group. This allows a `WorkSpaceTool` (for example) to have selection be a fallback tool if the user LeftMouse drags away from other gizmos in the group. Reviewed by: Campbell Barton (campbellbarton) Differential Revision: https://developer.blender.org/D15154