For Qt >= 6.5, the system theme can be determined reliably, so no
guesswork is needed. A fallback remains for Qt < 6.5, but it is
hacky and less reliable.
LibWebView now knows how to launch RequestServer and ImageDecoderServer
without help from the UI, so let's move ownership of these services over
to LibWebView for de-duplication.
We currently compile the Qt event loop files multiple times, for every
target which wants to use them. This patch moves these to LibWebView as
a central location to avoid this.
