[GTK] Fix printer enumeration in GTK4

gtk_enumerate_printers() doesn't always list all available printers in
GTK4 for some reason. However, the underlying backend getter
gtk_print_backend_find_printer() does work and it's what
GtkPrintUnixDialog uses internally, so this cl switches to using that.
Also the list of printers was not cached before, causing it to reload
the print modules on each invocation, leaking memory. This is fixed in
this CL by using a static variable to hold the print backend list.

R=thestig

Fixed: 414635403, 411336538
Change-Id: Idcd48c2fd7a95417004624131631c4255ec6c301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6513275
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457029}