QT_INLINE_SINCE

We have now had several requests for inlining previously-exported
member functions, but no standard mechanism to effect it.

As QT_REMOVED_SINCE has shown, there is great value in having such a
standard mechanism, so here is one.

Develop private API (= macros) to inline a previously exported (member) function,
by simply

• marking the declaration with QT_MODULE_INLINE_SINCE
• moving the definition into the header file (outside the class),
  • wrapping it in QT_MODULE_INLINED_SINCE, and
• #include'ing the header into the module's removed_api.cpp