Guard against get_selection() returning None.

The documentation doesn't mention anything about the possibility
of the method returning None, but who knows?

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@852 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Abort key event if a hotkey was triggered.

This is the default behavior of GTK+'s built-in accelerators, so it
would make sense in the custom manager as well.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@853 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Catch image changes to refresh preview of Edit Commands window.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@854 ef8490bc-f317-4725-81ed-e95a9aca1bcf
When probing for validy of executable, take working directory into account.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@855 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Show "Open" icon in menu bar next to "Open with" item.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@856 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Prevent blanking command labels.

At least one non-blank character is required.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@857 ef8490bc-f317-4725-81ed-e95a9aca1bcf