Character selection dialog and small UI changes.

Changed all dialogs so that they are contained within a Grid layout. I found this to work best for resizing. The widgets are now properly anchored to the dialog and will resize with the box.

I also edited the character selection to become an actual dialog box. It automatically loads up all .player files in the players directory and checks them for compatibility. It will then populate a list using that player's name if the file is compatible. Non-compatible files are ignored and if there are no compatible files then an appropriate dialog will appear.