Cancel background settings upon MatePanelAppletFrameDBus destruction.
This is an attempt to fix https://github.com/mate-desktop/mate-panel/issues/214
Resolve relative symlink of /etc/localtime
This fixes wrong detection of system timezone under certain circumstances.

Example: in Fedora 20, the environment is a follows:
a) /etc/localtime symlinks --> ../usr/share/zoneinfo/Europe/Zurich
b) /usr/share/zoneinfo contains files with more than 1 hardlink. In example,
   Europe/Zurich and Europe/Vaduz share the same i-node.
- system_timezone_read_etc_localtime_softlink() because the link is relative.
- The next algorithms fail because the targeted files do not exist.
- system_timezone_read_etc_localtime_hardlink() succeeds, but finds
  Europe/Vaduz before Europe/Zurich.
Thus the detected system timezone is wrong.

By resolving a relative /etc/localtime symlink, this patch leads algorithm
system_timezone_read_etc_localtime_softlink() to success and therefore proper
detection of system timezone.