Create Color Mode Menu
This adds a Color Mode submenu to the View menu. It makes the Change Colors feature of Okular easier to use.
The menu is a ToggleActionMenu, so it can be plugged into the toolbar.
The toolbar button will always show the last selected color mode, except Normal Colors, so the Change Colors feature can be toggled by just clicking it.
It also toggles when the selected action in the menu is selected again. But since Qt 5.14 introduced
ExclusiveOptional mode for
QActionGroup, this is probably an accepted UI behaviour.
This is originally taken from D21195.
(In other patches, less important if this does not go to the View menu)