This makes things work more similar to 2.79 until there is a toolbar for
mask editing. Select & slide is now always on the left mouse button.
For the LMB keymap the context menu is on the right mouse button. For the
RMB keymap the select & move is on the right mouse button, which is almost
the same as select & slide but for historical reasons there is still a
distinction.
Differential Revision: https://developer.blender.org/D5070
980726c1d4