Allow changing focus of lyrics edit via tab key

* This way it is no longer possible to insert a tab directly but that's
  likely not wanted anyways
* See https://github.com/Martchus/tageditor/issues/59

gui/tagfieldedit.cpp

@@ -338,6 +338,7 @@ ClearLineEdit *TagFieldEdit::setupLineEdit()
 ClearPlainTextEdit *TagFieldEdit::setupPlainTextEdit()
 {
     m_plainTextEdit = new ClearPlainTextEdit(this);
+    m_plainTextEdit->setTabChangesFocus(true);
     m_plainTextEdit->setClearButtonEnabled(true);
     m_plainTextEdit->addCustomAction(setupLockAction());
     m_plainTextEdit->addCustomAction(setupRestoreAction());