PERFORMANCE: Small functions are not all inlined when called from within Qt libraries

PERFORMANCE ISSUE/IMPROVEMENTS

To maintain compatibility when implementations change, Qt does not make small functions inline in public headers.

However, within Qt, small common functions like accesors could be available as inline implementations within private headers, to be used when called from other Qt libraries.

This could have a positive performance benefit., e.g. on ARM environments.

We should find candidate functions like this that use about 2 instructions for real work (ignoring stack/return operations), and start to make them inline within Qt (or if possible reduce the number of times they are called).

This can be done for any release, incrementally, with no API implications since it is internal to Qt.

We should use this issue to track the candidates and the changes we make.