[manjaro-settings-manager] Add a search field for finding entries

[rubyFeedback]

I tried to hit Ctrl+F for a find (search) widget, in manjaro-settings-manager
but could not find any.

I wanted to search for "font", in the hope of finding something there related
to fonts. Now I think fonts are not yet added to manjaro-settings-manager,
but irrespective of this, a search functionality that also gets tied to Ctrl+F
would be nice. GTK kind of has that where the search widget slides in, and
then you can type things there (in the gtk-entry). I guess qt or whatever
else is used here also has something similar related to finding something.

I think linuxmint also has that in their general settings widget.