fix #21398 - Update power plant related preset (patch by skyper)

* add `power=heliostat` to preset and style
* add a new group for `power=plant` with a general preset and one for each `plant:source`
 * multiselect with icons `plant:source` for general preset but no `plant:method`
 * correct `plant:method` per `plant:source` in individual plant presets
* `match="none"` to `generator:method` with only one value in `power:generator` presets, to have working presets links without `generator:method`
 * move the wiki links of `power:generator` up, to follow common practice.

git-svn-id: https://josm.openstreetmap.de/svn/trunk@18264 0c6e7542-c601-0410-84e7-c038aed88b3b