Bug 724692 - Canvas rotation stuck with specific order of actions.

The bug was affecting actually both canvas rotation and panning when
done with space key. If the first mouse button was also clicked, then
released after the space key, we ended up in some stuck action. It could
only be unstuck by hitting/releasing space again.

I am actually unsure that this was not originally done on purpose,
especially since the code has these 2 status variables space_pressed and
space_release_pending, but really apart from looking at this code, the
behavior just looks very buggy and impracticable.
The new behavior is to just stop the canvas panning/rotation as soon as
space is released (which is also how it is documented in our manual, and
how everyone seems to use the feature). I only kept the variable
space_release_pending, which I use as was used space_pressed before.

app/display/gimpdisplayshell.h

@@ -189,7 +189,6 @@ struct _GimpDisplayShell
 
   gboolean           inferior_ignore_mode;
 
-  gboolean           space_pressed;
   gboolean           space_release_pending;
   const gchar       *space_shaded_tool;