Inspector: Find themes the same way GtkCSSProvider does

Merged LRN requested to merge lrn/gtk:inspector-theme-find into gtk-3-24

Look for subdirectories named "gtk-3.x", where 'x' starts as current minor version and counts down to 14, then drops to 0. Only look for gtk.css in these directories though. If a theme only provides gtk-dark.css, it won't be found.

Merge request reports