Use a Solid::Device instead of its UDI

Makes things easier.
Disable back button and setup labels

Users cannot move backwards or forward from this page. The "Next" button
will be enabled once the installation is complete.
Emit completeChanged() signal

Emit the signal instead of calling it as method.
Set pixmap during the initialization phase

Move setup code to the initialization phase.
Hide the "Back" button and change text to the other buttons

This is the first page so it doesn't need the "Back" button, also "Next"
becomes "Install Maui" and "Cancel" becomes "Try Maui" to make it much
more clear what they do.