Client: Added commands for maximizing/centering the window

Also tweaked the output of "displaymode" to report the current,
windowed, and fullscreen sizes separately.
Client|libgui: Window resizing and moving

Fixed determining window width and height: should always use current
canvas size rather than the window's windowed-mode size.

Trying out a behavior where a centered window is re-centered if moved
only a little from its centered position.
libdeng2: Improved text representation of Rectangle<>
Fixed|Music: Expand symbols in the soundfont path
Fixed|libgui: Managing state of PersistentCanvasWindow

Todo: Check display mode changes.
Client: Tweaked output of the "displaymode" command