Quit menu item (Ctrl+Q) disabled-ness

[Akuli]

There is a "Quit" option in the "File" menu. It is disabled whenever you have one or more tabs open, because I found myself hitting Ctrl+Q accidentally when trying to hit Ctrl+W to close only the current tab. (This is especially annoying to me because I like to disable the restart plugin, meaning that I don't get previously open tabs restored when I open Porcupine again.)

A while ago @taahol mentioned to me that it is annoying how he can't use Ctrl+Q to quit Porcupine. I think we should add a checkbox "Ctrl+Q quits Porcupine only when there are no open tabs" or similar to the Porcupine Settings window. Instead of a hard-coded Ctrl+Q it should use whatever the user's key binding is, of course.