po: Update POTFILES.in with the latest changes in the Apps panel

The Apps panel now contains pages that used to be panels (Default Apps and Removable Apps).

This POTFILES.in update allows for translators to translate the strings in these files.

 Error regenerating template file (POT) for gnome-control-center:

xgettext --files-from POTFILES.in --directory /var/www/djamnedlies/data/scratchdir/git/gnome-control-center --from-code utf-8 --add-comments --output gnome-control-center.pot --keyword=_ --keyword=N_ --keyword=C_:1c,2 --keyword=NC_:1c,2 --keyword=g_dcgettext:2 --keyword=g_dngettext:2,3 --keyword=g_dpgettext2:2c,3 --flag=N_:1:pass-c-format --flag=C_:2:pass-c-format --flag=NC_:2:pass-c-format --flag=g_dngettext:2:pass-c-format --flag=g_strdup_printf:1:c-format --flag=g_string_printf:2:c-format --flag=g_string_append_printf:2:c-format --flag=g_error_new:3:c-format --flag=g_set_error:4:c-format --msgid-bugs-address https://gitlab.gnome.org/GNOME/gnome-control-center/issues
xgettext: cannot read /var/www/djamnedlies/data/scratchdir/git/gnome-control-center/panels/default-apps/cc-default-apps-panel.ui: failed to load external entity "/var/www/djamnedlies/data/scratchdir/git/gnome-control-center/panels/default-apps/cc-default-apps-panel.ui"

xgettext: cannot read /var/www/djamnedlies/data/scratchdir/git/gnome-control-center/panels/default-apps/cc-default-apps-panel.ui: failed to load external entity "/var/www/djamnedlies/data/scratchdir/git/gnome-control-center/panels/default-apps/cc-default-apps-panel.ui"

xgettext: warning: file 'panels/default-apps/cc-default-apps-panel.ui' extension 'ui' is unknown; will try C
xgettext: error while opening "panels/default-apps/cc-default-apps-panel.ui" for reading: No such file or directory