use systemwide xdg-open
util: don't fallback to gnome-open

we should only use xdg-open directly, which will invoke gnome-open on Gnome system itself.
It doesn't make sense to only support gnome-open as a fallback, because some would ask
'are we liking gnome more than others?' or 'why aren't you using kde-open as a fallback?'

so just to save us from such question we should drop gnome-open support
uicore: add google.com to internal URL whitelist

fixes #531
uicore: remove unneeded URLs from whitelist
Merge pull request #530 from karolherbst/xdg-open

use systemwide xdg-open and clean up URL whitelist