Add helpers to create lines

With this we can create either a horizontal or vertical line, and don’t need to manually set a constraint for its width/height.
Use horizontal/vertical line creators
Add move focus method

This moves focus to either the file browser to the text view (depending on which currently have focus).
Add Navigate → Move Focus menu item

This has ⌥⌘⇥ as shortcut so should make it easier moving to file browser and back.
WIP: Add action bar to file browser