Allow feature search to react to viewport and data layers changes

[Waguramu]

Currently, search is always run for a specific set of viewport tile-layers. This set does not change when the viewport is shifted or layers are (de-)activated. To avoid confusion for users, there should be a mode (which is on by default) where the search is re-executed/updated incrementally when the spatial/logical scope changes.

Note: Originally, this ticket also contained the following: Search doesn't incorporate coverage information - when I deactivate all data layers, any search finishes at once, if I activate one layer which has a coverage of one tile which is far away from my current viewport, it search all tiles.

This will be fixed by #145