[Core] Settings improvements: Grouping w/labels, support for doubles (#137)

* [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.

ObservatoryBotanist/BotanistSettings.cs

@@ -7,11 +7,12 @@ using System.Threading.Tasks;
 
 namespace Observatory.Botanist
 {
+    [SettingSuggestedColumnWidth(450)]
     class BotanistSettings
     {
         [SettingDisplayName("Enable Sampler Status Overlay")]
         public bool OverlayEnabled { get; set; }
-        [SettingDisplayName("Sampler Status Overlay is sticky until sampling complete (if enabled)")]
+        [SettingDisplayName("Status Overlay is sticky until sampling is complete")]
         public bool OverlayIsSticky { get; set; }
     }
 }