Add KHamburgerMenu
Current state of the hamburger menu contents:
Old video of the state of this MR prior to 20. August 2021: 2021-08-07_16-46-59
This MR does not affect the default look of Okular because the menu bar is visible by default.
This commit adds a hamburger button to the default toolbar of Okular. The toolbutton will be hidden by default but appear when the menu bar is hidden.
This new menu button uses KHamburgerMenu from KDE frameworks. It acts as a substitute for the menu bar if users prefer not seeing the menu bar.
This commit also replaces the "Show Menubar" action that appears in the right-click context menu of the main view when the menu bar has been hidden with a "Menu" button that is mostly identical with the hamburger menu but does contain the "Show Menubar" action and an action to show the main toolbar. Through this change there is still a way for users to recover a menu even if they have hidden all of them but with the added benefit that a very minimal usage of Okular without any bars is now possible.