Lots of newer apps that use their appid as their icon name don't set
window icons, since they aren't used in GNOME. Instead of setting it
manually in every app, just default to it.
Only set the icon if it exists in the icon theme.
Remove manually set default icons in the demo.
No tests as GtkApplication doesn't have any in the first place.
Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/7120
There's no benefit in having multiple windows share the process.
But there's a huge disadvantage because running the app a 2nd time with
different environment variables will open a window in the first process
instead and discard the variables.
And my use of GSK_RENDERER hates that.
We want to use the same header in the inspector, so move it to
the toplevel. And since it is no longer for demos only, rename
it to profile_conf.h, and also rename the build option back
to profile.
Add a Help item to the gear menu that opens the
node-format.md file in a new window. This could
be improved if we could parse markdown and apply
tags, similar to how we can load pango markup.
