Assign key equivalents to Go → Back/Forward

We use ⌘[ and ⌘] which is also used for Text → Shift Left/Right so we only set these keys for when the main text view is not active.

The main advantage is that you can now use these keys to move back and forth in history of HTML views (it also works in file browser, but due to a custom keyDown: overload, it also worked prior to this commit).
Add Edit → Select → None (⇧⌘A)

This can be used to deselect all in the file browser.

Closes #779.