Add a few functions to plugin utilities.
Fix bug in PluginsLoader.load function.

If a plugin was enabled but later removed from disk then the program
would crash. KeyError exception occured in

plugin = self.all_plugns[UID]

finally block tried to use  finding_failed variable which was not
defined because it was initialized after the above line. This
was causing another exception that was not caught.

Added logger and some log messages.

Added handler for KeyError exception.
Fix untrack call in PluginsLoader.

untrack function was called with plugin object but were supposed
to be called with UID. This was crashing the program because in
some cases plugin did not exist.

Gave PluginsLoader logger object.
Add functions for inserting actions in the context menu.

At the moment there is only function for adding to the leaf menu.
Merge pull request #19 from alexey-naydenov/plugin_helpers

Utilities to help with plugins