add ghost function getAllDisplayDimensions, GHOST_GetAllDisplayDimensions

This returns the desktop size, not just the size of the active monitor, useful since this constrains the mouse and we dont have to detect the active monitor (which isn't so straightforward with xlib).

carbon/cocoa are TODO, they still use getMainDisplayDimensions().


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54226 954f8c5b-7b00-dc11-b283-0030488c597c