Ask for roof levels if not set

[Discostu36]

Use case
At the moment, building levels and roof levels are asked in the same quest. This leads to the problem that if a building already has building:levels added but no roof:levels, StreetComplete will not ask about the roof levels and also not about the roof shape.

Proposed Solution

Two possibilities:

1. If a building does have a building:levels tag but not a roof:levels tag, show it in the building levels quest, pre-fill the building levels field and put the cursor into the roof levels field.
2. Add a separate roof level quest for buildings missing the roof:levels tag

[HolgerJeromin]

SC does not set roof:levels if not given (and does not ask roof:shape).
So this behavior is as designed.
#1744, #1778, #277