Optimize feature requests when identifying

[uclaros]

When identifying on the map canvas, be it single tap or tap and hold, a feature request is performed which fetches all attributes of all nearby features . Most of the times this does not matter, but if a layer has an expensive attribute (eg virtual field with relation_aggregate expression) then the issue is apparent.

This can be optimized by fetching only what is needed, and only fetch the whole feature when we need to display its form:

• single tap: FID and geometry are only needed for the proximity check
• tap and hold: fields used in the display expression are only needed (no geometry)