window: Differentiate test instances from normal ones

When running a test instance alongside the regular Polari instance,
it is easy to lose track of which is the temporary one. Address this
by adding some subtle styling hints in the test instance's title bar.

Fixes #24
1 job for wip/fmuellner/debug-instance in 5 minutes and 5 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-polari #8489

00:05:05