Add ad hoc debug backtracing of selected hooks

[bobbingwide]

There are times when you want to know what's invoking a particular action hook or filter.

Proposed solution

Add Hooks to debug trace textarea field to identify the hooks where you want to produce debug backtrace output each time the hook is invoked.

For each hook listed the trace output will produce a trace record using bw_trace() and a debug backtrace using bw_backtrace().

The implementing function will be called bw_trace_backtrace()