* [Core] Settings improvements: Grouping w/labels, support for doubles
Layout improvements:
* Plugins can set `[SettingSuggestedColumnWidth(123)]` on a settings class to adjust the settings view column width to make it wider or narrower to fit things nicely.
* Plugins can set `[SettingNewGroup("label")]` on any setting property to force a "new paragraph", or group, of settings. If "label" is also provided, a grouping header with that text will also be created.
* A double precision up/down numeric control has been added. In support of this, there is now a precision value on the existing `[SettingNumericBounds]` attribute to specify the number of digits of precision the control displays/allows.
Plugins:
* The above have been applied/demonstrated on the Botanist and Explorer plugin settings.
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