selectionmodel: Change prototype of query_range()

selectionmodel: Change prototype of query_range()

1. Do not make position an inout variable
The function is meant to return a range for a given position, not modify
a position. So it makes no conceptual sense to use an inout variable.
2. Pass the selected state as an out variable
Using a boolean return value - in particular in an interface full of
boolean return values - makes the return value intuitively feel like a
success/failure return. Using an out variable clarifies the usage.
3. Allow passing every position value
Define what happens when position >= list.n_items
4. Clarify the docs about how this function should behave
In particular, mention the case from point (3)
5. Add more tests
Again, (3) needs testing.