IDC_Overlay cleanup

This MR is unfortunately quite large. I will try to summarise what was done:

1. IDC_OverlayBase package was reorganised a little bit

   • IDC_OverlayCommon contains only static methods from the Overlay namespace
   • IDC_OverlayBase class has only member functions. Due to some issues, it is still basing on OverlayAlgBase.

2. PixelOverlay, SCTOverlay and TRTOverlay have been updated for the changes.

3. MDTOverlay has been updated and cleaned up to be closer in the naming to other overlay algorithms.

Note that the TRT overlay algorithm will be additionally cleaned-up in a follow-up MR so I plan to fix all typos and inconsistent naming later.

The output does not change in the test case although Pixel unit tests had to be changed.

/cc @jchapman