Dynamic screen resolution change support

[trunkmaster]

The goal of forthcoming set of changes is provide better multi-monitor support to GNUstep. This part brings XRandR extension support to:

• receive and handle XRandR screen resolution change events;
• send NSApplicationDidChangeScreenParameters notification;
• retreive and return screen dimensions from XRandR structures (boundsForRect) instead of Xorg display's;
• use boundsForRect instead of DisplayHeight() - it returns old height of screen after screen pixel dimension changes.

The next important change is update origin of windows in new screen dimensions - libs-gui PR #50.

[trunkmaster]

Actually current screen handling implementation is limited. NSScreen percieves screen as a display/monitor device. Other side - backend - manipulates X11 screen - the sum of displays bounds (overlapped or placed side by side). Saying that, I plan to change this part of -back -gui interconnection. Meanwhile I merge this PR and start making enhancements as new PR.