[ssu] Expose new dbus API for updating the store credentials.

[jolla] Resolve store credentials from store-client via dbus

Fix sandboxing

Replace legacy code which does not use the Sandbox class.

Otherwise existing (not-sandboxed) `ssud' instance is talked to and
system configuration is destroyed during test execution.

"All" where it makes sence.

Forward declare QDomDocument

Saves other packages from including QT+=xml in their project file until
actually needed.

[ssu-ks] allow to override default ssu parameters.

[ssu] Warn if registration fails due to non-writable configuration

/usr/share/ssu/kickstart/pack/<model>, with default fallback

Recursive search used to return the first match found, now return the last match
found. Additionally, fall back to an autodetected default section if required.