Add a dialog that allows replacing missing sensors

When a page is missing sensors, it may end up not displaying correctly. This can happen because we have certain sensors that are bound to physical hardware, which disappear or do not exist when we use a page from the store.

Rather than trying to do a difficult (probably impossible) mapping between sensors or trying to prevent uploading pages with hardware-dependent sensors, provide a UI so this situation can be resolved by the user.

Depends on libksysguard!229 (merged)

Message:

Dialog: