HC: Copy working tooltip code from Adwaita & tweak

HC: Copy working tooltip code from Adwaita & tweak

There were various problems, like only selecting on .tooltip and not the
widget node tooltip, not being specific enough for tooltip.csd, etc. So,
specific theming was absent, and default popup window styles got applied

This commit copies in the better working tooltip CSS from Adwaita, but
applies a couple of changes to make it work better in the HC themes:
 • Reduce the transparency of the tooltip, so we achieve higher contrast
 • Drop the black text-shadow, as it is not useful on this more black bg

Note: we may then need to re-add some of this to the .tooltip class. But
it is unclear what needs done there. While Adwaita is not doing it, we
are better not to confuse by keeping it in HC only; we should try to be
as close as possible, to make it easier for HC to keep up with Adwaita.

https://bugzilla.gnome.org/show_bug.cgi?id=769879