[MWF] Implement multi-display support on Linux (Fixes #325669)

The number of displays and sizes can be retrieved with the help of
libXinerama. The implementation was done in a way that is backwards
compatible, so on non-Linux systems or when libXinerama is not
available the behavior will be the same as before.

Also fixed the implementation of FormStartPosition.CenterScreen to
bring up the form centered on the current display. This fixes
Novell bug#325669 on Linux
(https://bugzilla.novell.com/show_bug.cgi?id=325669).