Profilers controllable via PYQTGRAPHPROFILE.

A new function profiling system is implemented.  Most importantly, this
allows one to profile various internal functions directly by setting the
`PYQTGRAPHPROFILE` environment variable to a comma separated list of
function and method names, e.g.

    PYQTGRAPHPROFILE=functions.makeARGB,ImageItem.render \
        python -mexamples

Specifically, items in `PYQTGRAPHPROFILE` must be of the form
`classname.methodname` or `dotted_module_name.functionname`, with the
initial "pyqtgraph." stripped from the dotted module name.

Moreover, the overhead of inactive profilers has been kept minimal: an
introspective check of the caller's name (only if `PYQTGRAPHPROFILE` is
set) and a trivial function (not method) call per profiler call.

The new profilers rely on `sys._getframe` to find the caller's name,
although the previous system (passing the caller's name explicitely)
could certainly have been kept instead.

Finally the API of profilers has been changed: register a
profiling point simply by calling the profiler, and profilers are
automatically flushed on garbage collection.  See the docstring of
`pyqtgraph.debug.Profiler` for more details.