Allow the user to choose a directory to save a copy of virtualtilesets.vts.

As with *.tiles files (which go in the game directory).
Display and select the TileShape when double-clicking a virtual tile.
Fixed color of translucent virtual tiles (windows, grime, etc).

The trick was passing QGLContext::PremultipliedAlphaBindOption to bindTexture().
Converting the image to premultiplied beforehand does not work.

Experimented with simple shader programs that also gives the desired result.
Fixed window title of TexturePropertiesDialog.