Parse global config files. Remove 'Vendor default' option. Fix changes not recognized.

The fonts kcm did only parse a local config file. Global and default settings were not recognized.
With this patch, all global config files are parsed before the local file is parsed.
This allows us to drop the "Vendor default" option.

This patch also fixes some changes not recognized (apply button disabled).

CCBUG: 386566

Test Plan:

Delete ~/.config/fontconfig/fonts.conf and ~/.config/kcmfonts .
Run kcmshell5 fonts. The default/global settings should be displayed.
Change some settings. The apply button should be enabled.
Save. A new config file should be created.

Follow up of the D12925