Dev:Ref/Proposals/UI/Full lmb support

Allow the user to use LMB select efficently

When a user changes their selection method from right-click to left-click in the User Preferences, the RMB becomes pretty much useless.

Therefore, Blender should allow users to turn on right-click menus in the User Preferences as well.

Also, clicking with the left mouse button in empty 3d space should deselect all objects.