StClipboard: add st_clipboard_get_mimetypes

Since the `MetaSelection` of the global `StClipboard` is private (and
should be), there is no reasonable way for extensions or external code
to query the supported mime-types.

Add `st_clipboard_get_mimetypes()` so this can be queried without
poking around in private code.

closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2819

src/st/st-clipboard.c

@@ -146,6 +146,29 @@ transfer_cb (MetaSelection *selection,
   g_free (text);
 }
 
+/**
+ * st_clipboard_get_mimetypes:
+ * @clipboard: a #StClipboard
+ *
+ * Gets a list of the mimetypes supported by the default #StClipboard.
+ *
+ * Returns: (element-type utf8) (transfer full): the supported mimetypes
+ */
+GList *
+st_clipboard_get_mimetypes (StClipboard     *clipboard,
+                            StClipboardType  type)
+{
+  MetaSelectionType selection_type;
+
+  g_return_val_if_fail (ST_IS_CLIPBOARD (clipboard), NULL);
+  g_return_val_if_fail (meta_selection != NULL, NULL);
+
+  if (!convert_type (type, &selection_type))
+    return NULL;
+
+  return meta_selection_get_mimetypes (meta_selection, selection_type);
+}
+
 /**
  * st_clipboard_get_text:
  * @clipboard: A #StCliboard

src/st/st-clipboard.h

@@ -65,6 +65,9 @@ typedef void (*StClipboardCallbackFunc) (StClipboard *clipboard,
 
 StClipboard* st_clipboard_get_default (void);
 
+GList * st_clipboard_get_mimetypes (StClipboard             *clipboard,
+                                    StClipboardType          type);
+
 void st_clipboard_get_text (StClipboard             *clipboard,
                             StClipboardType          type,
                             StClipboardCallbackFunc  callback,