qml: hide the artists pane resize handle if it is not functional

When `VLCStyle.isScreenSmall` is true, the artists pane can
not be resized. However since the horizontal resize handle is
still visible, the cursor changes and this makes it confusing
for the user whether the pane can actually be resized or not.

modules/gui/qt/medialibrary/qml/MusicArtistsAlbums.qml

@@ -316,8 +316,10 @@ FocusScope {
                 sourceWidth: root.width
                 targetWidth: artistList.width
 
+                visible: !VLCStyle.isScreenSmall
+
                 onWidthFactorChanged: {
-                    if (!_inhibitMainCtxUpdate)
+                    if (!_inhibitMainCtxUpdate && visible)
                         MainCtx.artistAlbumsWidthFactor = widthFactor
                 }