Added customizable pen colors

New node property, penColor, is null by default; this means that color
will be picked using the existing plotColorPalette method.
New node context menu item, Pen color, lets user choose the color to be
used when plotting this particular node.
This property is also saved/loaded from settings based on sensor
identifier.

GUI/MainForm.Designer.cs

@@ -111,6 +111,7 @@ namespace OpenHardwareMonitor.GUI {
       this.log1hMenuItem = new System.Windows.Forms.MenuItem();
       this.log2hMenuItem = new System.Windows.Forms.MenuItem();
       this.log6hMenuItem = new System.Windows.Forms.MenuItem();
+      this.colorDialog = new System.Windows.Forms.ColorDialog();
       this.splitContainer.Panel1.SuspendLayout();
       this.splitContainer.SuspendLayout();
       this.SuspendLayout();
@@ -762,6 +763,7 @@ namespace OpenHardwareMonitor.GUI {
     private System.Windows.Forms.MenuItem log1hMenuItem;
     private System.Windows.Forms.MenuItem log2hMenuItem;
     private System.Windows.Forms.MenuItem log6hMenuItem;
+    private System.Windows.Forms.ColorDialog colorDialog;
   }
 }