[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.

ObservatoryExplorer/ExplorerSettings.cs

@@ -1,12 +1,14 @@
 using Observatory.Framework;
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 
 namespace Observatory.Explorer
 {
+    [SettingSuggestedColumnWidth(300)]
     public class ExplorerSettings
     {
         public ExplorerSettings()
@@ -14,6 +16,12 @@ namespace Observatory.Explorer
             CustomCriteriaFile = $"{Environment.GetFolderPath(Environment.SpecialFolder.Personal)}{System.IO.Path.DirectorySeparatorChar}ObservatoryCriteria.lua";
         }
 
+        [SettingNewGroup("Display")]
+        [SettingDisplayName("Only Show Current System")]
+        public bool OnlyShowCurrentSystem { get; set; }
+
+
+        [SettingNewGroup("Built-in Criteria")]
         [SettingDisplayName("Landable & Terraformable")]
         public bool LandableTerraformable { get; set; }
 
@@ -83,12 +91,9 @@ namespace Observatory.Explorer
         [SettingDisplayName("High-Value Body")]
         public bool HighValueMappable { get; set; }
 
+        [SettingNewGroup("Custom Criteria")]
         [SettingDisplayName("Enable Custom Criteria")]
         public bool EnableCustomCriteria { get; set; }
-
-        [SettingDisplayName("Only Show Current System")]
-        public bool OnlyShowCurrentSystem { get; set; }
-
         [SettingDisplayName("Custom Criteria File")]
         [System.Text.Json.Serialization.JsonIgnore]
         public System.IO.FileInfo CustomCriteria {get => new System.IO.FileInfo(CustomCriteriaFile); set => CustomCriteriaFile = value.FullName;}