Replace qDebug with a real logging framework

I'm currently interested in this one https://github.com/gabime/spdlog

Here's a snippet from the README:

Very fast (see benchmarks below).
Headers only or compiled
Feature rich formatting, using the excellent fmt library.
Asynchronous mode (optional)
Custom formatting.
Multi/Single threaded loggers.
Various log targets:
    Rotating log files.
    Daily log files.
    Console logging (colors supported).
    syslog.
    Windows event log.
    Windows debugger (OutputDebugString(..)).
    Easily extendable with custom log targets.
Log filtering - log levels can be modified in runtime as well as in compile time.
Support for loading log levels from argv or from environment var.
Backtrace support - store debug messages in a ring buffer and display later on demand.

I'm particularly interested in the custom formatting parts, the console output, log filtering, and backtrace support.

I've implemented a testing version in #37