This patch adds support for:

• Per space-type tools (3D view and edit).
• Per mode tools (object, edit, weight-paint .. etc).
• Multiple active combinations of space-type & mode at once.

What Works

• Toolbar in image and 3d view.
• Switching modes changes tools.
• Topbar only shows last activated tool (this is a design issue, since topbar is global and tools are per space).

Not Yet Working

• Multiple windows multiple, different tools with manipulators *(currently unlinking manipulators isn't aware of other workspaces windows)*.

Plans

• Remove WM_OT_tool_set, using an operator here causes quite a few jumps between C and Python in the callstack. Some of this logic can be kept in Python's WM_OT_tool_set_by_name.
• Improve the image toolbar (currently it's a proof of concept to show whats possible).
• Refactor toolbar definitions, one file per space-type, share code in common location.

Notes

• Having per-space-type API calls: workspace.tools.from_space_view3d_mode / from_space_image_mode isn't nice. It's needed because the values of the mode argument depend on space_type.
• WM_toolsystem_update_from_context_view3d from mode switching operators are needed because we cant use message bus subscribers to run operators or set RNA values (needed for switching tools).