Allow positive matching for match_expression attribute in preset item

[Woazboat]

Currently the match_expression attribute for preset <item> only allows you to further restrict matches for presets. (i.e. exclude things that are matched by fields in the preset but not by the match expression)

Allowing the item match expression to act as a positive match would unlock greater flexibility for presets.

For example, ​crossing ways can be tagged either as ​footway=crossing, ​cycleway=crossing or ​path=crossing. All of these have the same additional tags that could be used to further describe them (e.g. ​crossing:markings=*). At the moment, we would need to create three separate presets with the same content to be able to show a matching preset for each of these tags (one each with <key key="footway" value="crossing" />, <key key="cycleway" value="crossing" />, ...). It's also possible for these crossing ways to have both footway=crossing and cycleway=crossing tags if the crossing is a shared foot- and cycleway. In this case, both matching presets for footway=crossing and cycleway=crossing would be shown.

If the match_expression attribute on <item> was able to positively match objects, a single preset for these crossing ways could be created via <item type="way" match_expression="footway=crossing | cycleway=crossing | path=crossing" ...>

[Woazboat]

Among the default presets, only the generic presets for power plants, power generators and route network relations use match_expression to disable themselves if a more specialized preset could better describe the feature. These 3 presets are the only users of match_expression in the core.

[skyper]

Replying to Woazboat:

If the match_expression attribute on <item> was able to positively match objects, a single preset for these crossing ways could be created via <item type="way" match_expression="footway=crossing | cycleway=crossing | path=crossing" ...>

As the key of the optional secondary tag depends on the value of the primary tag highway=*, I would not use one preset for all but rather add the additional tags to the existing presets or create new ones for each. Note, you can use <chunk /> for identical tags used in several presets.