applets/showdesktop: vary labels instead of checkbox & fix Minimize All active title
The Peek at Desktop and Minimize All Windows applets have context menu entries that allow accessing either function; they currently use checkable menu entries. This makes the entries look like settings, rather than actions the user can trigger, leading to confusion.
This change removes the checkable property and instead uses the already existing action labels as the context menu entries. Icons are also set to fill the space previously used by the checkboxes.
The Minimize All action when active (i.e. the action will undo Minimize All" currently says "Restore All Minimized Windows". This can be misleading, as undoing the "Minimize All" action is not the same as un-minimizing all windows; windows that already were minimized prior to using Minimize All will (and should) stay minimized.
This changes replaces the sometimes incorrect title with the unspecific but accurate "Restore Windows". The translation contexts are also updated to more concretely reflect how these strings are actually used in the UI.
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