Fix a lot of high-contrast inconsistencies or brokenness. Refs #315

* In a couple of places I had to resort to if(IsHighContrast) but mostly
  this is just using system brushes consistently or not assuming black
  text.
* The default DockPanel theme doesn't work well, so make a minimal high-
  contrast theme for it and assign it everywhere.
* The pipeline flow was using fixed colours, use system brushes for the
  different elements and switch based on high-contrast to ensure active
  and inactive stages are visible (using ActiveCaption looks bad on
  normal themes because it's a big block of colour).
* For some reason the flat toolstrip renderer doesn't handle white-on-
  black themes, but the system one does. It's a little clunkier but it
  shows up correctly without writing tons of custom painting code.
* Range histogram uses a properly contrasting colour for the border.
* Treelist views use a better system colour for selected rows when
  inactive and hovered rows (when high contrast).
* Mesh view grids have a system background instead of white
* Various things (pipeline state, mesh viewe) set text colour when
  colourising backgrounds of things instead of assuming black.

renderdocui/Windows/Dialogs/PythonShell.cs

@@ -30,6 +30,12 @@ namespace renderdocui.Windows.Dialogs
         {
             InitializeComponent();
 
+            if (SystemInformation.HighContrast)
+            {
+                toolStrip1.Renderer = new ToolStripSystemRenderer();
+                toolStrip2.Renderer = new ToolStripSystemRenderer();
+            }
+
             shellTable.Dock = DockStyle.Fill;
             scriptTable.Dock = DockStyle.Fill;