Increase Qt requirement to 5.12 and remove pre-5.12 workarounds

app/gui/NavigableMenuItem.qml

@@ -2,10 +2,6 @@ import QtQuick 2.0
 import QtQuick.Controls 2.2
 
 MenuItem {
-    // Qt 5.10 has a menu property, but we need to support 5.9
-    // so we must make our own.
-    property Menu parentMenu
-
     // Ensure focus can't be given to an invisible item
     enabled: visible
     height: visible ? implicitHeight : 0
@@ -15,7 +11,7 @@ MenuItem {
         // We must close the context menu first or
         // it can steal focus from any dialogs that
         // onTriggered may spawn.
-        parentMenu.close()
+        menu.close()
     }
 
     Keys.onReturnPressed: {
@@ -27,6 +23,6 @@ MenuItem {
     }
 
     Keys.onEscapePressed: {
-        parentMenu.close()
+        menu.close()
     }
 }