Add a singleton for managing xsettings overrides

https://bugzilla.gnome.org/show_bug.cgi?id=705359
UI changes, new categorization of the tweaks
some tweaks were moved to the tweak_legacy.py file
tweak_legacy.py is loaded with the  -l command line option.
Notifications with the logout option
using GDBus for the logout
using a GtkStack to store/show/hide the tweaks of each category
The GtkStack can not be homogeneous
Single instance of gnome tweak tool
the search works
some changes
remove old css override
add some padding inside the main window
remove the obsoleted search entry manager widet
separators between rows
a new main window
Add GSettingsCheckTweak
Desktop icons are GSettingsCheckTweaks
Increase padding in the search entry
Move CSS to external file

* set a list row background color
* set a minimum width for the sidebar