UI|Client: Added a tutorial dialog

Todo: Widget for binding the console key.
libappfw|Added: Special button with a small auxiliary button inside

AuxButtonWidget has an auxiliary button inside that can be used for
extra functionality.
Fixed|libappfw|LabelWidget: Updating label's margins
libdeng2: Added more character symbols (Alt)
libappfw|libgui: Key modifier related cleanup
UI|Bindings: Added widget for making an input binding

The bindings subsystem was enhanced with functions for finding
existing bindings for a command and deleting bindings based on
the bound command.

InputBindingWidget currently supports only command bindings for
the keyboard device.
Tutorial: Added console key binding widget

The tutorial now offers an opportunity to change the console shortcut
key. This is only shown when a game is loaded, because in Ring Zero
bindings are currently not persistent.
UI|Console: Added menu item for setting console shortcut key