preferences: fix error on saving domain configuration

Don't check for CHANGES_DONE_HINT before saving the user-edited domain configuration. Checking this temporarily changes the path of the file that is being edited, appending an "~" to the path (maybe this is a Vala bug?). Here's how it looked like when I logged the filename:

(gnome-boxes:2): Boxes-WARNING **: 19:23:11.966: resources-page.vala:254: /var/home/pedro/.fedora36-wor-3.draft. txt
(gnome-boxes:2): Boxes-WARNING **: 19:23:19.674: resources-page.vala:254: /var/home/pedro/.fedora36-wor-3.draft.txt~

This ended up leading to a "No such file or directory" error every time the user saved their custom domain configuration.

The downside to this change is that the domain configuration is now saved everytime the user writes to the .txt file, instead of waiting for the likely last change in a set of changes.