Add disconnect button in main menu

I've added disconnect button in main menu. There is one in multiplayer menu, which doing same, but when player plays in singleplayer mode, to disconnect he must open multiplayer menu.

In this commit I've add disconnect dialog (cause I'm new at QuakeC, I make it based on dialog_quit.qc/qh), add it to mainwindow at the bottom of screen, between credits and quit. I haven't idea how to make dialog visible in Nexposee, there is not attributes for this. Thats reason why I create 2 variables inside - dialog variable and variable of visibility. In draw event I'm checking state of current visibility of dialog and add/remove element from nexposee. With variable of visibility, I'm guarantee that adding and removing dialog will be only one time when changing game state.

There are some screenshots:
Main menu when not in game,
Main menu when in game,
Disconnect dialog (looks same as quit)