GtkFileFilter: Allow case-insensitive patterns

In the past, we've hardcoded case-insensitive matching
on Windows, and case-sensitive matching everywhere else.

With this change, we now have a way to explicitly add
case-insensitive patterns to a filter. We still interpret
all patterns case-insensitively on Windows.
4 jobs for ci-file-filters in 25 minutes and 1 second (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #112399
fedora-x86_64

00:05:58

passed #112400
win32
msys2-mingw32

00:09:19

 
  Flatpak
passed #112401
flatpak:demo

00:15:41

passed #112402
flatpak:widget-factory

00:15:39