[kcm/mobilewifi] Rework actions

The current usage of Kirigami actions has some problems.

We do not use Kirigami actions in other KCMs. They only work in plasma-settings, but not in systemsettings. Not a huge deal in this particular case though. Using the main action for dis/enabling wifi always felt a bit weird to me. There was no clear indication what the button does There is also lacking some indication that wifi is disabled.

This patch:

• Adds a placeholdermessage that informs that the wifi is disabled and offers to enable it
• Adds a footer with more actions, including disabling the wifi. This is where we usually put actions in a KCM