Made maya keymad deselect everything in cases nothing was selected with LMB

The way it works now is:

- Border select is using TWEAK, for which you need to hold LMB down
  and move mouse. This prevents operators using RELEASE/CLICK events
  from being fired.
- LMB select is using RELEASE event, this is no selection happens
  before border if you do border select. And this prevents any
  operator uses CLICK event from being fired.
- Delect all happens by CLICK, which would be fired only in case no
  operators were handled with PRESS/RELEASE.

This is a bit cards-house, but this is how events currently works and
wouldn't want inventing something bigger now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54535 954f8c5b-7b00-dc11-b283-0030488c597c