Better light theme (Style) support, and a Default light theme (as a secondary option)

1) Added ForegroundViewport as a new color.  It is used in the main game viewport (ViewportWidgetButton), and the viewport for rendering of particles and materials.  It is needed because the default foreground in a Light theme is black, but black does not work well in a viewport.  A new color seemed appropriate.

2) Fixed the profiler window to use the Foreground color in multiple text elements, instead of Color.White (or no default TitleColor).  This includes  the Row class, Asset class, SingleChart class, Timeline Class, and more.

3) Added a second theme/Style (DefaultLight) to include with the engine.  It uses RGB float values because those were easier to transfer from the saved values that I had created (and they're easier for me to edit if necessary).  I tried to emulate how the Default theme is created/loaded/etc as closely as possible.

Source/Engine/UI/GUI/Style.cs

@@ -104,6 +104,12 @@ namespace FlaxEngine.GUI
         [EditorOrder(110)]
         public Color ForegroundDisabled;
 
+        /// <summary>
+        /// The foreground color in viewports (usually have a dark background)
+        /// </summary>
+        [EditorOrder(115)]
+        public Color ForegroundViewport;
+
         /// <summary>
         /// The background highlighted color.
         /// </summary>