Aspect ratio / Undo / misc bug fixes

- Added aspect ratio correction checkbox to Gfx viewer and texture
editor. This stretches images vertically by 20% to give a preview of
in-game proportions for certain types of graphics.
- Undoing will now first query the entry panel first. For now, only the
text editor has a built-in undo feature.
- Text editor no longer allows to undo loading the entry...
- Init opened time in SLADEMap's constructor so that SLADE will not
randomly query to save a nameless map when quitting without having
opened the map editor at all.
- Updated a bit stuff.txt.