Keyboard shortcut to toggle filter search

[mgunyho]

Add keyboard shortcut to toggle filter search, Alt-f by default.

• [x] Add action & keyboard shortcut for it
• [x] Fix cursor behavior when the toggle happens
• [x] Fix cursor behavior when gap search is changed
• [x] Fix cursor behavior when case sensitive mode is changed
• [x] Update README
• [x] Update changelog

Fixes #59

[mgunyho]

This isn't ready yet, the cursor position doesn't get correctly updated when toggling. For example, see the following situation:

tere-filter-search-toggle.webm

The key sequence is a, down, alt-f, esc. This is the same as the test test_filter_search_toggle in app_state.rs.

Will probably require a bit of work to get this behavior right.

Upon initial investigation, I think the problem might be that app_state.get_item_under_cursor() behaves incorrectly if the filter mode has been changed.