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It was lost during the markdown conversion.

Discovered-by: Timm Bäder <mail@baedert.org>

https://bugzilla.gnome.org/show_bug.cgi?id=721309

Split out function for setting attributes. This will be needed in future
patches.

Only create an attribute list for merging if we actually need to merge.

This bug was introduced in 5230cfe8

The margin-start and margin-end properties were incorrectly
defined, so margins weren't respected for RTL languages.

https://bugzilla.gnome.org/show_bug.cgi?id=723627

Add background and border, so the popover boundaries are visible.