GS 3.38 prefs support

[fawtytoo]

This extension works with GS 3.38 too.

But the prefs don't:
`Gtk.BuilderError: .:12:43 Invalid property: GtkViewport.scroll-to-focus

Stack trace:
PreferencesDialog@/home/steve/.local/share/gnome-shell/extensions/burn-my-windows@schneegans.github.com/prefs.js:37:19
buildPrefsWidget@/home/steve/.local/share/gnome-shell/extensions/burn-my-windows@schneegans.github.com/prefs.js:226:16
_init@resource:///org/gnome/Shell/Extensions/js/extensionsService.js:207:40
OpenExtensionPrefsAsync/<@resource:///org/gnome/Shell/Extensions/js/extensionsService.js:122:28
asyncCallback@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:132:22
run@resource:///org/gnome/Shell/Extensions/js/dbusService.js:177:20
main@resource:///org/gnome/Shell/Extensions/js/main.js:19:13
run@resource:///org/gnome/gjs/modules/script/package.js:222:19
start@resource:///org/gnome/gjs/modules/script/package.js:206:8
@/usr/share/gnome-shell/org.gnome.Shell.Extensions:1:17`

[Schneegans]

Thanks for the report, I am well aware of this (#2, #3, #5). The next version will contain a GTK3 port of the settings dialog.

[Schneegans]

I just released Burn-My-Windows 3 which adds official support for GNOME 3.36 and 3.38.