Open Find bar instead of toggling it on Ctrl + F

<kbd>Ctrl</kbd>+<kbd>F</kbd> currently toggles the Find bar. I.e. when the Find bar is already activated, pressing Ctrl + F hides it again. This behaviour is inconsistent with what most if not all other applications do: Ctrl + F should focus the find field when it's used while the find bar is open.

This PR corrects the behaviour. If the user wants to close the find bar, s/he has to use <kbd>Esc</kbd> instead.