Add standard shortcut F10 for "Open Main Menu"

The standard shortcut "Create Folder" is changed away from F10 to Ctrl+Alt+N.

A consistent/standard way to "Open Main Menu" is needed so blind or otherwise visually-impaired users are able to open the general menu. Currently there is no consistent way for blind users to open a menu. There is no generally working way to open the menu bar using only the keyboard. Alt+F tends to work in the English language because most applications with a menu bar also have the "File" menu as the first menu. But this is neither true for all application that have a menu bar, nor is it true that every application has a menu bar in the first place.

Applications with a hamburger menu do generally not provide a shortcut to open it either. This new standard shortcut can be used to provide a consistent way so people are able to discover the features of an application no matter which kind of main menu an application provides.

F10 for this is consistent with Gnome's HIG, Microsoft's "common keyboard shortcuts", Firefox and Chromium. There doesn't seem to be any other choice than F10 to provide a consistent user experience.

Unfortunately F10 was already used as the "Create Folder" shortcut until now. This commit changes that. Instead Ctrl+Alt+N is used because Ctrl+N is typically used to create a new window or a new file (in file creation software). The "N" standing for New. Ctrl+Shift+N is used for a modification of this i.e. creating a new "special" window or restoring a previously closed window. When we use Ctrl+Alt+N for creating a new folder, the Alt makes clear that this is not just a modification of the Ctrl+N shortcut but about creating something different. A folder in this case. This reasoning is not necessarily super convincing or set in stone but it is why this was chosen.