Integrate snapd-xdg-open into snapd repository

Hi,

our app should be able to open any type of file.

Basically, it does the following:
When the user click on the name of one content downloaded the following code is executed in a child process:
xdg-open path_to_file

See the following screenshot, in this case the user clicked in the “Download the video script” link that it is a pdf file that was downloaded a few days ago. The app will execute xdg-open and the OS will open the PDF viewer (in this case the Ubuntu one):