Make unfocused terminal text transparent instead of blacker

[julienfalque]

I noticed that the option to change the text brightness of unfocused terminals actually makes the colors tend to black instead of becoming transparent as I thought it would. This is not a big deal with a dark background as the result is more or less the same (depending on the exact background color):

However the result is a bit weird on a light background:

I think the result would look better with actual transparency instead. Maybe related to #74.

[mattrose]

This is basically #74. It may actually get fixed when we go to GTK4. Migrating the codebase is a huge PITA, but it does a lot of things better.