Make primary-selection unknown when not supported

This lfun will not appear in menus anymore in macOS and Windows.

Part of bug #12436.

src/LyXAction.cpp

@@ -3417,7 +3417,7 @@ void LyXAction::init()
 /*!
  * \var lyx::FuncCode lyx::LFUN_PRIMARY_SELECTION_PASTE
  * \li Action: Pastes the currently selected text.
- * \li Notion: Primary selection mechanism is linux-only thing.
+ * \li Notion: Primary selection mechanism is X11/Wayland-only thing.
  * \li Syntax: primary-selection-paste [<ARG>]
  * \li Params: <ARG>: "paragraph" will cause pasting as one paragraph, i.e. "Join lines".
  * \endvar

src/Text3.cpp

@@ -3437,6 +3437,7 @@ bool Text::getStatus(Cursor & cur, FuncRequest const & cmd,
 		break;
 
 	case LFUN_PRIMARY_SELECTION_PASTE:
+		status.setUnknown(!theSelection().supported());
 		enable = cur.selection() || !theSelection().empty();
 		break;
 

src/frontends/Selection.h

@@ -27,6 +27,11 @@ class Selection
 public:
 	virtual ~Selection() {}
 
+	/**
+	 * Returns true if the underlying system supports mouse selection
+	 * (basically X11 and Wayland).
+	 */
+	virtual bool supported() const = 0;
 	/**
 	 * Tell the window system whether we set or cleared our selection.
 	 * This is a noop on systems that don't have a selection.

src/frontends/qt/GuiSelection.h

@@ -34,6 +34,7 @@ public:
 	/** Selection overloaded methods
 	 */
 	//@{
+	bool supported() const override { return selection_supported_; }
 	void haveSelection(bool own) override;
 	docstring const get() const override;
 	void put(docstring const & str) override;