[WIP] Add search capability

[m8]

This pr introduces search capability for thingy, it searches on recent and favorites without an app_id.

While using add_document_to_library function i passed app_id as a null string. Please let me know if this should be change.

[mtwebster]

Hi, a few things:

• See my inline comments
• You're lower-casing the search string, but not the strings you're comparing against in the filter function, so currently I can't find a file called "UPPER_FILE.txt" with either "UPPER" or "upper".
• Clearing the search entry, either by clicking the X icon, or by backspace, etc... should return to the full file list, not blank.
• It would probably be most useful to have the entry as the focus when the program starts. So a person could immediately start typing without having to click in the entry first.

Thanks!

edit: You might want to consider using a filter for the flow box, instead of rebuilding the view each time:

https://lazka.github.io/pgi-docs/#Gtk-3.0/callbacks.html#Gtk.FlowBoxFilterFunc https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/FlowBox.html#Gtk.FlowBox.invalidate_filter