Improve GUIDropDown auto-closing behaviour and appearance

[mjn33]

Hello, this patch aims to improve the behaviour of the GUIDropDown control, it seems it is becoming more widely used in nuFAR and I though it could do with a few changes:

• The drop-down is positioned under the button used to activate it, and will not close if the mouse if over that button. Should make it less likely to disappear because you moved the mouse 2 pixels from where you clicked.
• The drop-down now renders the scrollbar and text consistently with the rest of the KSP skin.
• Cleaned up the code to be a bit more self-documenting.
• The currently active item's button text is now highlighted.

Preview:

[ferram4]

Oh, cool! Thanks, this is much appreciated.