desktop: add %U to Exec command

so xdg-open desura:// will result into desurium launching/installing the game
Revert "A message now pops up if Desura is already running."

This reverts commit 5dd2abf4c7667e2619c1dbe1029c1183791497b7.

fixes #515

there is no need of printing a message at all
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 #529 from karolherbst/bug515

fixes Bug515
Merge pull request #530 from karolherbst/xdg-open

use systemwide xdg-open and clean up URL whitelist