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
Merge pull request #529 from karolherbst/bug515

fixes Bug515