Engine: rewrote gfx mode automatic selection

This finds acceptable resolution and gfx filter simultaneously instead of making several attempts to init gfx mode with given filter.
If the certain gfx filter was requested by user, but no acceptable gfx mode found, engine automatically calculates maximal supported nearest-neighour filter.
In contrary to original feature, this change allows more freedom when choosing resolutions.
There's still certain restriction to the size of borders, imposed by the way letterboxing is implemented in AGS currently.
Engine: implemented free letterboxing

This removes hard-coded letterboxing parameters and allows letterboxing for any game resolution.
Engine: removed obsolete code related to gfx mode
Engine: proper error message if auto mode search failed
Game resolution is now enum
WinSetup: updated letterbox option logic
Editor: deprecated LetterboxMode setting
WinSetup: updated UI positions and option names

This positions gfx borders related options together and names them in a more elaborate way.
Engine: ignore game's default letterbox setting

This forces engine to only use letterbox setting from configuration file and ignore the property stored in game data.
Engine: fixed letterbox mode for 320x240 and 640x480 games
Manual: updated references to letterboxed mode
Merge pull request #147 from ivan-mogilko/update_scaling

Improved range of supported display modes