Add a GUI

[dennis95]

We should be able to run multiple graphical applications at the same time rather than just one fullscreen application.

[dennis95]

This would also give us something interesting to show in screenshots (#18).

[00p513-dev]

Maybe X Windows could be ported? Of course once #22 #23 and #24 are fixed

[dennis95]

I have created a little GUI demo in the gui-demo branch.

It supports multiple windows, transparency, resizing, and each window has a working close button.

What is still missing to make the GUI useful, is the ability for applications to create their own windows inside the GUI and a way for the GUI to forward events to those applications. That can be implemented once sockets (#23) exist.

[dennis95]

The gui branch now contains a working GUI compositor.
Applications can create windows within the GUI and receive events about key presses, mouse movements, etc.
Further changes will be required before it is ready to be merged into the master branch.

[dennis95]

The GUI has now been merged into the master branch.