Improvements to callback and stack trace frame function analysis

[johnbillion]

Various improvements to the handling of callbacks (which may or may not be a valid callable) and stack trace frame functions. Currently these are handled the same way, but they're not the same thing so shouldn't be.

Aims

• Introduce distinct handling for different types of stack traces, stack trace frames, and callback instances
• Deprecate QM_Backtrace in favour of a new stack trace collector which uses less memory
• Make a distinction between classes which should be ignores in the stack trace and database driver classes, the latter of which should remain in the stack but should not be taken into consideration when determining the caller

Traces and callbacks

• Hook callbacks
• Block render callbacks
• Fatal error stack trace frames
• Call stack trace frames