fix problem that Pidgin3 couldn't be run using broadway GDK_BACKEND anymore
Due to changes in r/2338 running Pidgin in broadway resulted in following error:
plugins-Message: 00:30:55.180: Auto-loading plugin /home/grim/p/pidgin/build/libpurple/plugins/kwallet/purplekwallet.so
qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Aborted (core dumped)

So this patch reverts back to QCoreApplication. To deal with the runtime warning r/2338 was trying to solve, we now install a message handler that puts the Qt warnings and other messages into Pidgin's debug window.

ran Pidgin3 using broadway as GDK_BACKEND
checked that the kwallet warning ("Cannot use KWindowSystem without a QGuiApplication") appeared in the debug window when switching credential manager to kwallet