xdg-settings: adapt to KDE changes

In https://gitlab.freedesktop.org/xdg/xdg-utils/-/blob/master/scripts/xdg-settings.in#L214:

The script checks with text/html but Kde plasma now stores the default browser as the associated app to "x-scheme-handler/http".

In https://gitlab.freedesktop.org/xdg/xdg-utils/-/blob/master/scripts/xdg-settings.in#L239

This function is wrong nowadays, plasma uses the application associated with "x-scheme-handler/http" as default browser if no BrowserApplication is set in kconfigglobals.

Basically, KDE plasma is now closer xdg specification. https://phabricator.kde.org/D17371