Add Lava SVG (issue #32)
Remove all occurences of TYPE_MASSIVE, etc. Closes #42.

There are no static images displayed anymore. This is probably
wrong.
Restore static sprites on XML loading.

There are still problems with front passive sprites.
Add a warning for undefined sprite types
Fix loading/saving of (front)passive sprites.

Z order is still wrong.
Rename cSprite_Manager::Set_Pos_Z() to cSprite_Manager::Ensure_Different_Z() and explain what it does.

And make it private. This is not meant to be called from outside
the sprite manager. Also fix it to properly work with massive
types instead of general types.

Currently, this causes enemies to be given Z coordinates in the same
line as other sprites, so that e.g. flyons appear before their pipe.
Don’t give enemies Z coordinate correction.

You should not place enemies over one another, really.

Now the flyons stay in their pipes :-).
Merge branch 'remove-massivity-from-sprites' into devel