Console|libdoomsday: Rudimentary access to de::Config variables

This commit introduces a simple mechanism that allows the user to
interactively access de::Config variables as if they were traditional
cvars. The allowed operations are checking the value of the variable
(having it printed in the console), or setting the value (number or
text only).

This is not a long term solution, though. When the console is based on
Doomsday Script, this mechanism should become unnecessary and obsolete.
(Although some form of mapping between traditional cvar identifiers and
Config variables is still needed.)
Cleanup
libgui: Querying the ID of a window
Refactor|Client|libappfw|libgui: Replaced 'vid-fsaa' and 'vid-vsync' with Config variables

Console variables are game-specific, however the user most likely
prefers to use the same FSAA/vsync settings for all games as their
appropriate values depend on the available hardware.

Now the FSAA and vsync settings are configured persistently using
Config.window.main.fsaa and Config.window.main.vsync. The old cvars
were removed, however they can still be accessed via the console
command line using a mapping.

IssueID #1832
Fixed|ClientWindow|Windows: Use of the new fsaa/vsync variables
Debug|libgui|GLTarget: Converted a GL assert to a debug message

On Windows, this assert may fail during canvas recreation.

Todo: In the long run, make canvas recreation unnecessary. Hopefully
Qt 5.4 will help here.