gs-info-bar: Minor styling fixes

This makes the infobar look not completely broken. There’s more work to
do, but that probably involves more in-depth redesign of the use cases
for these infobars in the first place.

The aim of this commit is just to make them look not completely broken.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #197
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