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+Shift+N is used
because Ctrl+N is typically used to create a new window or a new
file (in file creation software) and Ctrl+Shift+N is the closest to
this when Ctrl+N is already assigned to something. Ctrl+Shift+N is
also used in non-KDE file managers for folder creation.

The only argument against Ctrl+Shift+N was that this should be
reserved for opening new "special" windows. Ctrl+Alt+N was
considered instead but overruled. No other keyboard shortcut came
to mind either.