Button icons once more

Three issues with icons for VC buttons:

1. The icon file was read on every repaint
2. All icons were "disabled" in design mode and thus drawn in grayscale. Now only "Blackout" and "Stop all" are gray in design mode.
3. Icons in workspace file were stored with full path, so moving the workspace file to another computer broke it. The paths are now stored relative to workspace location. Tested only in Linux.