linux-xshm: Add server property.

This adds a property to the source properties which lets the user
specify the X server to capture from. Since this is probably something
thats only useful under certain circumstances it is implemented as
an advanced setting which is only shown when the corresponding option
for advanced settings is checked.
linux-xshm: Handle missing screens.

This adds a check to the screen enumeration in order to handle missing
screens by adding them to the list but disabling them.