Multi-monitor setup support

[Sergii Stoian]

Hi,

Yesterday I've submitted 2 PRs: https://github.com/gnustep/libs-back/pull/18 and https://github.com/gnustep/libs-gui/pull/50. These changes provide ability for running apps adopt monitor configuration changes (resolution and layout).

Actually this is first little step to multi-monitor setup support. I want to end up with set of enhancements to GNUstep:

- Application awareness of monitors parameters (resolution, layout, depth, DPI) changes. Application should update it's windows positions and autosaved frames.

- Application/window awareness when window moved across monitors.

- Application menu should move to key window's monitor (move window, switch key state between windows on different monitors).

- Correct window position restore from autosave frame. Now windows sometimes moved down relatively to the top part of desktop.

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Sergii Stoian