Fab for filter action

[veniosg]

Add a fab that on click will show a text field on its left.
Typing in that field will cause the list view to filter out any items not matching the typed text, in real time.

A map of filter queries to paths should be retained for the files up to the current one. When navigating to any directory we should be clearing all entries of that map referring to paths of a level deeper or equal to the newly navigated-to file. Backing out of the app (on finish of FileManagerActivity) should clear the map.

A temporary, barebones UI should suffice for this as that can be polished independently.
The mechanism for filtering is already in place in the file filter.
This functionality should be covered by UI tests.