Implement implicit sharing for QgsField

[nyalldawson]

Implements implicit sharing for QgsField. I'm seeking comments on whether this is a correct implementation of Qt's implicit sharing classes, and if so, will continue to switch additional classes to be implicitly shared.

[jef-n]

typeName

[m-kuhn]

If you want to experiment with implicit sharing, go for QgsFeature, it would be great to be able to copy this around without having to care much :)

[nyalldawson]

@m-kuhn agreed, but I wanted to start with something straightforward as a trial...

[m-kuhn]

@nyalldawson fair enough.
Concerning the implementation, is there no need to provide code for the operator=() method?

[nyalldawson]

@m-kuhn I think QSharedData/QSharedDataPointer takes care of that automatically. I'm struggling to find definite answers either way...

[m-kuhn]

I think it may be good to define FieldData in a separate header file that is not considered public API. This should reduce compile time and maybe even runtime loading performance. If I am not mistaken we should be able to profit from that: https://gcc.gnu.org/wiki/Visibility (at least our windows builds as I think we don't use visibility stuff on other platforms)

PS: Please don't expect that I completely have understood the article in every detail...

[nyalldawson]

@m-kuhn how's this? It probably needs some extra cmake changes to keep the _p.h header private too...

[m-kuhn]

Looks good to me. Maybe the private header could somehow be removed from install/devel-packages. But I don't think it hurts apart from inflating them.

[nyalldawson]

manually merged

[wonder-sk]

Great stuff :-)