menubar: add 'up' button, fix back/forward behavior
The commits here are pretty atomic.
Bookmark button is moved into the menu.
Navigation buttons are grayed out when unusable.
Up button is added (see gemini://mozz.us/journal/2021-01-01.gmi), bringing you to the parent page.
Since only 3 buttons can fit there, if we want to add 'root' page functionality in the future, it should be a multi-use feature of the up button.