* feat: let a collection name the attribute its modes switch by
Manually switched modes were always selected by data-<collection name>, so a collection called New wrote data-new and a codebase already using data-color-scheme could not be matched. A collection can now name its attribute in the inspector, validated so a stylesheet can select it as is. The stylesheet and exported code use it, setting it is undoable, and .fig files keep it in the collection's plugin data.
* fix: show the switch attribute only where there are modes to switch
* fix: return to the list after committing the switch attribute
* fix: keep a collection's switch attribute when a new one is refused
setModeAttribute cleared the attribute when given a name a stylesheet cannot select on; it now leaves the current one, and only an empty name restores the default. In the inspector, Enter on a refused name keeps the focus so it can be corrected.
* docs: describe the switch attribute in the mode conditions
The Switched manually row still said the attribute is named after the collection. The changelog had collected four copies of the variables dialog entry through rebases; one remains, with the switch attribute in it.
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