Add missing parent for messagebox and other popups

Unparented popups can easily get lost in backround, but they still block top level event loop when run with ::exec() thus preventing interactions with main window.  This mainly happens on wayland. Setting the parent ensures they are always kept on top and reasonably positioned.

src/Mod/TechDraw/Gui/CommandExtensionDims.cpp

@@ -113,7 +113,7 @@ void execInsertPrefixChar(Gui::Command* cmd, std::string prefixFormat, const QAc
 
     std::string prefixText(prefixFormat);
     if (prefixFormat.find("%s") != std::string::npos) {
-        DlgTemplateField ui;
+        DlgTemplateField ui(Gui::getMainWindow());
         const int MAX_PREFIX_LENGTH = 31;
 
         if (action) {