feat(ui): style app menus and extract glossary dialog

- Add AppMenu and AppMenuItem so menu labels, mnemonics and shortcuts render consistently under org.kde.union
- Move the translation glossary into GlossaryDialog with add, remove and clear-all actions
- Regroup the translation preset and instructions settings layout

src/qml/SubtitleTracksMenu.qml

@@ -9,7 +9,7 @@ import QtQuick.Controls
 import org.kde.ki18n
 
 // Checkable list of subtitle tracks, for the primary or the secondary subtitle.
-Menu {
+AppMenu {
     id: root
 
     required property MpvItem mpv
@@ -36,7 +36,7 @@ Menu {
 
     enabled: mpv.subtitleTracks.length > 0
 
-    MenuItem {
+    AppMenuItem {
         text: KI18n.i18nc("@item:inmenu no subtitle", "None")
         checkable: true
         checked: root.selectedId === 0
@@ -49,7 +49,7 @@ Menu {
 
     Instantiator {
         model: root.mpv.subtitleTracks
-        delegate: MenuItem {
+        delegate: AppMenuItem {
             required property var modelData
             text: root.trackLabel(modelData)
             checkable: true