Add option to hide menubar

 * Xviewer version (3.4.4)
 * Distribution (LMDE 6 Faye)

Issue
Xviewer doesn't have an option to hide the menubar. Xreader does, and it'd be nice to have that option here too.

Steps to reproduce

1. Open Xviewer
2. Click the 'View' menu
3. Note lack of 'Menubar' option

Expected behaviour
A 'Menubar' checkbox would be available under the 'View' menu (checked by default).

[wengyk]

Since there is no function to hide the menu bar, I can only use eog instead of xviewer.

[Ginoan]

I just want to add support that I'd like to see this either fixed or added.