[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 (with the current
display being either the screen the top left corner of the owner
form is on, or if there is no owner the screen that has the mouse
pointer). This fixes Novell bug #325669 on Linux
(https://bugzilla.novell.com/show_bug.cgi?id=325669).
[MWF] Fix Screen.FromRectangle (#8271)

The Screen.FromRectangle method was too simple minded.  It did not try to
find the best match for a screen when the system has more than one screen.
The new implementation makes at least some effort to find the closest
screen with the most overlap with the rectangle.
[MWF] Fix ComboBox list to not display offscreen (#15462)

The fix mimics the behavior of Windows/.NET.