Implement PreferencesPanel.

I have created a preferences panel that will be
one of the pages of the main windows notebook.
Then new panel is a Gtk.Stack and includes the
same behaviour that the previous PreferencesDialog had.
Old PreferencesDialog classes are deleted.
Move search bar from main windows to a new class.

When the search text changes, a signal is thrown
containing information about the new search parameters.
Implement edit panel.

Edit panel contains a messages list and a
hint list widget. Related deprecated classes
are removed from the repository.
Implement toolbar.

Main windows headerbar has been moved to a
separated class where we allow it to have
different forms.
Reimplement search.

I have created a class that receives the
information about a running search. I have modify
the seach class constructor to receive the edit pannel
where it has to find the results.
Modify several files and classes to make the previous commits work.