Undo: implement undo of tag editing

The code follows the other edit-commands, but uses its own base
class, because it is distinctly different. Editing the tag field
does not simply mean setting the tag for all dives, but rather
adding and removing individual tags.

This class will be reused for editing of dive buddies and masters.

Modify the tag widget thus that it sends an editingFinished()
signal when it goes out of focus. The editingFinished() signal
was prevented by hooking into the return, enter and tab key-events.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/tagwidget.cpp

@@ -211,3 +211,11 @@ void TagWidget::fixPopupPosition(int delta)
 		m_completer->popup()->setGeometry(toGlobal.x(), toGlobal.y() + delta +10, toGlobal.width(), toGlobal.height());
 	}
 }
+
+// Since we capture enter / return / tab, we never send an editingFinished() signal.
+// Therefore, override the focusOutEvent()
+void TagWidget::focusOutEvent(QFocusEvent *ev)
+{
+	GroupedLineEdit::focusOutEvent(ev);
+	emit editingFinished();
+}