qt: Prevent non-functional GUI from popping up during Init

When a InitError or InitWarning happens, the
GUI pops up but is unusable (until Init finishes).

This is caused by showNormalIfMinimized. Add a message
flag to skip this call for Init errors or warnings.

src/ui_interface.h

@@ -62,6 +62,8 @@ public:
 
         /** Force blocking, modal message box dialog (not just OS notification) */
         MODAL               = 0x10000000U,
+        /** Don't bring GUI to foreground. Use for messages during initialization */
+        NOSHOWGUI           = 0x20000000U,
 
         /** Predefined combinations for certain default usage cases */
         MSG_INFORMATION = ICON_INFORMATION,