Use Caja GSettings schema optionally

This was the last place where it was used unconditionally.
Merge pull request #239 from City-busz/make-caja-optional

Use Caja GSettings schema optionally
Update Marco WM name as per Marco commit

Closes https://github.com/mate-desktop/mate-panel/pull/240

 cbf24f5d78719b930fc0cd6c0c80e00e9d62c95a
Do not assume that session manager GSettings schema is available

This removes hard dependency on mate-session-manager (useful for custom environments).

Closes https://github.com/mate-desktop/mate-panel/pull/241