Fixes #3

Fixes #1

https://bugzilla.gnome.org/show_bug.cgi?id=774176

The others borders in the css are displayed because inherit the css
class from frame. That doesn't happens with chat-sidebar, so we have
to explicitely use border-style and border-color.
Also, a typo prevented the header to have border.

GtkTextView's horizontal size request will grow over time
when added to a GtkScrolledWindow with horizontal scrolling
disabled, even when the TextView is set up to wrap lines.

As a workaround, we overwrite its size request with a constant
width to force the text to wrap around.

See 65ce0de0e3abc9de4fd672c31a92120bf548c518 commit in polari.

Relying on GtkFrame's shadow for borders will give us additional
vertical borders that are already covered by the sidebar's border;
to avoid double borders, explicitly set the borders we actually need.

The value for OnlyShowIn must end with a semi-colon.