* [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.
Polishing a few rough edges:
* Enable scrolling on the Plugin List view. It only fit 6 or 7. I have 13.
* I fiddled a bit with column auto-sizing (left it commented out, see comment).
* A whole bunch of tweaks to the settings view:
* first row of controls was cut off at the top (under the window title bar)
* fixing that revealed uneven spacing between columns
* action buttons were teensy tiny and unreadable
* settings view columns were too narrow (200 px).
* added some spacing to help with readability
* settings windows now start in center of current screen rather than some random place.
