Panelize: preserve orientation of component references / values on silkscreen

[romavis]

This is a known issue covered in FAQ: https://github.com/yaqwsx/KiKit/blob/master/doc/faq.md#i-rotated-my-board-in-a-panel-and-the-component-references-did-not-rotate
The proposed solution (uncheck "keep upright" on all reference labels) surely works, but it takes quite some time to do that, and disabling that option also complicates editing original PCB layouts, before they're panelized.

Expected behavior

• When I'm laying out each one of my boards, I usually want "keep upright" to be enabled, so that labels always take 0 or 90 degrees orientation, irrespective of component orientation. So when I finish the layout of the board, I know exactly how it would look like, including the silk screen. This is the default behavior of pcbnew.
• When I'm panelizing those boards, and rotate some of them, let's say, by 180 degrees, everything within the board should rotate by 180 degrees: tracks, zones, footprints, drawings, and, of course, silk screen labels. If on the panel I have two copies of the same board, one with 0 degree orientation, and one with 180 degree orientation, after depanelizing the boards they should look the same.

How can that be achieved?
I've quickly tested a simple approach: within appendBoard(), if a text label of a component (reference or value) has "keep upright" attribute set, we calculate actual orientation of the label before component has been rotated, remove "keep upright" attribute, and rotate the label so that it keeps its original orientation. Afterwards, when the component is rotated, labels will be rotated by the same amount.

I'd like to hear your opinion on this @yaqwsx. I can implement the necessary code change for 5.1 branch, and this behavior can be made optional (e.g. enabled only when a certain optional argument of appendBoard() is set to True).

[yaqwsx]

Having this implemented would be definitely welcomed. There were, however, tasks on KiKit that had much higher priority for me so I rather implemented them. Therefore, if you submit a PR I will be happy to merge it. Once we have this, I don't see a point in having this behavior enabled by a switch - I cannot think of a use case when you would like to rotate the designs but keep the labels upright.