feat: add option to hide menubar

[pesader]

I added a "View" submenu to the menubar, that allows the user hide the menubar. I also added a keyboard shortcut (Ctrl + M, as suggested in issue #1251) to toggle the visibility of the menubar. This preference is saved, so it can be restored once the user closes and reopens Zeal.

This preference is tipically only accessible in the "View" submenu, so I didn't add an entry for it in the "Preferences" window. I could do that as well, if you prefer.

I haven't been able to show/hide the menubar when Alt is pressed/released though, so this isn't ready. I decided to create this draft PR anyways, in case someone else knows how to that, since it would solve issue #1251 and make progress towards solving issue #1202.

https://user-images.githubusercontent.com/65264536/201760101-b4d640a0-7b9b-42a1-a3cd-3df964d084e5.mp4

[trollixx]

Thanks for the contribution! I've been meaning to implement this by copying behavior from Firefox. The menu bar is not very useful as #1251 rightfully stated. I'd go further and hide it by default. Then rely on F10 to either show and focus the menu bar, or just focus when it's visible. Adding a submenu in a menu with a single item may seem to be an overkill, but that's where zoom options for #1256 should also go. Would you be willing to update PR with a similar UX?

[pesader]

Hey there! Of course, I'd love to contribute with that. I'm busy with my final exams right now, but as soon as that's done I'll come back to this :)