libdeng2: Added the Property utility for observable native variables

Unlike script Variables, Property deals with native value types
and cannot accept more than one type of value.
Console: Disabled warning about ‘playdemo’

Demos are currently not supported, so no point in warning about them.
libgui|GLFramebuffer: Use a Property to track the default sample count

All existing GLFramebuffer instances observe the default sample count
and update their contents accordingly when it’s changed.
Fixed|UI: Use GLFramebuffer when drawing a blurred widget background

The game renderer assumes that both color and depth attachments are
being used.
libdeng2: Added a header for some printable character symbols

Unicode if possible.
Log|Resources|libgui: Improved log output