Refactor: Added libgui, separated TextApp and GuiApp

libgui extends libdeng2 with GUI-related code. libdeng2 itself no longer
depends on QtGui, only QtCore and QtNetwork.
Client|Server: Dropped GUI/input dependencies in server binary

The server has no input devices, and therefore needs no bindings either.
Both the input and bindings subsystems were dropped from the server.

The text-mode console was dropped altogether from both client and
server builds (to be replaced with the Shell).

The window manager was dropped from the server, which is now a TextApp.
Server: Must not depend on OpenGL