Refactor orientation management in pyFAI-calib2 widget

[kif]

Follow up of #2020

You should create a dedicated class for the orientation. You can feel like it is overkill, but that's the way not to be over bloated with implementation details after years.

• set/get will deal with your enum values, you don't have to think about
• you can put your component inside detector-selection-drop.ui directly

[kif]

Some related issues:

• [ ] #2174: make some nice sketches to explain the difference
• [ ] #2176: offer the option to flip horizontally an image in a silx image
• [ ] #2177: new a marker for the detector origin
• [ ] #2175: update the code in calib2