Add import/export for R16 heightmaps

These are used for games. The format is undocumented, but basically
just a stream of 16 bit integers.
Also add support for r8 heightmap files

Note: the common extension for 8 bit raw heightmap files is .raw, however
that conflicts with panasonic .raw raw camera files. Since panasonic
registered the mimetype for .raw and the games people didn't, the games
people lose and have to mess about with another extension, i.e., .r8.
Fix compilation

The transaction is not needed while loading anymore
Disable Video Shape for everything except Stage
[FEATURE] Edit Selection Mask

This patch adds an action

Selection->Show Global Selection Mask

When the checkbox is toggled, it makes the global selection masks be
shown in the Layers docker. When you activate this mask, you can use
any tool you like on top of it. For example Transform or Brush tools :)
bump recommended version of SQLite to 3.8.5