This adds a button to the file layer which, if clicked will attempt to open the file that the file layer is referencing.

This is useful for users so they can quickly access a file layer.

This also fixes several issues with relative paths and file layers.

I build and ran it. There's a small bug in that this new property shows up in the properties dialog, but fixing that will require writing a properties dialogue for the file layers, which'll need a bit more work.