Make a new global QgsGui singleton

[nyalldawson]

This PR implements a new QgsGui global singleton, which itself contains members replacing the other existing gui singletons.

I.e. instead of QgsEditorWidgetRegistry, QgsShortcutsManager, etc singletons, instead we have a global QgsGui singleton with these instances as members.

This is driven by:

1. Ongoing removal of singleton effort. While this approach DOES introduce a new singleton, it avoids some of the issues with the current isolated singleton approach by allowing us to manually construct and destruct the members in a logical order.
2. There's a need for some form of gui singleton. Add OS native interface lib, with objective-c++ interface to Mac Cocoa ibraries #3964 is blocked by the need for some global container within gui to contain the native interface class/QObject.

[m-kuhn]

Are there open questions regarding this?
I think the approach is fine in general 👍

[nyalldawson]

Are there open questions regarding this?

Only the name really. I'll merge for now and it'll be easy enough to rename later if we decide on a better one.