Enh applet-coherence: Show different icon when coherence is running.

This is part of issue #339. Thanks to PhobosK for the patch.
Enh applet-coherence: Honour user's prefs file (~/.coherence) if exists.

This is part of issue #339. Thanks to PhobosK for the patch.
Make applet-coherence executable and fix she-bang-line.
Security fix: FSStore exports whole user-dir if passed no pass at all.

When an empty content is passed to update_config() in FSStore backend
(for example when all content is removed by the export widget in
av_widgets.py), the os.path.abspath("") expands to the user's home
folder thus exporting all his folders and files there. This is totally
unacceptable and is now fixed.

This solves one part of issue #338. Thanks to PhobosK for the fix.
Security fix: FSStore by default shares Music, Videos, Pictures folders.

The problem is that the user is not aware of this behaviour and these
folders could contain sensitive materials that the user does not want
to share.

Solution: It is much better to use as default share folder the
XDG_PUBLICSHARE_DIR. At least the user knows it is shared and he does
not keep sensitive materials there.

This solves the other part of issue #338. Thanks to PhobosK for the fix.
Fix she-bang line: call python via /usr/bin/env.

Prior to this change the interpreter's path  was hardcoded. Thus in a
virtualenv, the wrong interpreter was used if the script was called
directly.