Rename Plugin & API namespaces and implement saving/loading of setting

src/DesktopMagicPluginAPI/Settings/TextBox.cs

@@ -1,4 +1,4 @@
-namespace DesktopMagicPluginAPI.Settings;
+namespace DesktopMagic.Api.Settings;
 
 /// <summary>
 /// Represents a text box control.
@@ -31,4 +31,14 @@ public class TextBox : Setting
     {
         Value = value;
     }
+
+    internal override string GetJsonValue()
+    {
+        return Value;
+    }
+
+    internal override void SetJsonValue(string value)
+    {
+        Value = value;
+    }
 }