special handling for global bbox in xml queries

global bbox parameters in xml queries uses an OverpassQL-like
notation instead of n/s/e/w parameters. This needs special
treatment to not create invalid queries. Introduced some regex
magic for this.

Example:
  <osm-script bbox="{{bbox}}">
becomes
  <osm-script bbox="1,2,3,4">
expose codeEditor for better testability
add tests for query expansion
fix "empty dataset" detection for the case where only <bounds> are returned