FEAT(client): Introduce new logging system based on spdlog

[davidebeatrici]

Sinks:

• OutputDebugString() (for debugger on Windows)
• Standard output stream (stdout)
• File
• Developer console (Qt widget)

Improvements compared to our old logging system based on Qt:

• Duplicate messages are skipped if less than 5 seconds have passed. When that happens, an informative message is printed.
• The previous format/pattern is retained, but the log level (e.g. <W>) is colored based on the severity:
  
• The log is now written to a file on all platforms, not only Windows and macOS.
• When the log file's size reaches 5 MiB, the sink renames it and creates a new one. Up to 4 files are kept:

  Console.txt   -> Console.1.txt
  Console.1.txt -> Console.2.txt
  Console.2.txt -> Console.3.txt
  Console.3.txt -> <delete>
  

• We can eventually create multiple loggers, ideally one for each facility (Audio, ServerHandler, etc.).
  That would allow us to quickly filter log messages and know right away which subsystem is responsible for them.
• Modern C++ string formatting. This of course only applies to the new functions such as log::debug(), not qDebug().

Only one "feature" is not reimplemented: showing a dialog in case of a fatal error, right before exiting the program.
However, that only worked on Windows and macOS using their native API, to avoid depending on a Qt event loop:

mumble/src/mumble/os_win.cpp

Lines 83 to 86 in 5452eef

	if (type == QtFatalMsg) {
	::MessageBoxA(nullptr, qPrintable(msg), "Mumble", MB_OK | MB_ICONERROR);
	exit(0);
	}

mumble/src/mumble/os_macx.mm

Lines 52 to 57 in 5452eef

	if (type == QtFatalMsg) {
	CFStringRef csMsg = CFStringCreateWithFormat(kCFAllocatorDefault, nullptr, CFSTR("%s\n\nThe error has been logged. Please submit your log file to the Mumble project if the problem persists."), qPrintable(msg));
	CFUserNotificationDisplayAlert(0, 0, nullptr, nullptr, nullptr, CFSTR("Mumble has encountered a fatal error"), csMsg, CFSTR("OK"), nullptr, nullptr, nullptr);
	CFRelease(csMsg);
	exit(0);
	}

[davidebeatrici]

For reference, the Windows build failed just for this:

D:/a/1/s/src/mumble/Logger.cpp(55): error C2220: the following warning is treated as an error
D:/a/1/s/src/mumble/Logger.cpp(55): warning C4459: declaration of 'masterSink' hides global declaration
D:/a/1/s/src/mumble/Logger.cpp(30): note: see declaration of 'masterSink'