Improve keyboard navigation for macOS

[BPerlakiH]

Based on the PR discussion here: #1079

The navigation in the app using keyboard only, can be improved using the focus engine.

Although the PR itself is suggesting a keyboard short cut to the search field, that is not something I would consider to be implemented at this point, since the search bar is contextual. We have one search bar to find content in ZIM files, one for searching bookmarks, and two for searching categories (selected category or the "new").

Additionally to that the user can be in a place where non of those search bars are visible, eg: the "Opened" tab, where the user can browse the already downloaded / local ZIM files.

For these reasons a global keyboard short cut is not appropriate.

Luckily though, the focus can be moved around the application with the TAB button, and with a couple of focus improvements, a similar effect can be achieved.

Note: It has to be a macOS only implementation, without affecting iOS.