Add keybindings to show the menubar temporarily when hidden

Closes #47 (closed), #68 (closed).

This MR addresses #47 (closed) and therefore reproduces a common behavior for the menu bar nowadays, when it is hidden (e.g. for Firefox which I used as a reference):

• When the Alt key is released, show the menubar temporarily (i.e. settings are not affected);
• When the Alt or Esc keys are then pressed, hide the menubar;
• Hide also the menubar after some user actions (mouse buttons pressed/released, scroll, focus out);
• When a menubar mnemonic key is activated (e.g. Alt+f) show the menubar and popup the corresponding submenu (then hide the menubar and its submenu all at once).

This led me to refactor a bit the code around menubar/toolbar/statusbar display in window/fullscreen mode.

I also integrated @lastonestanding's proposal regarding default keybindings in #68 (closed).