qtlog.disabled() and some bug fixes

• Fixed log line number in messages, and provide better contextual information in Qt5 (#55, thanks @The-Compiler);
• Fixed issue where exceptions inside a waitSignals or waitSignal with-statement block would be swallowed and a SignalTimeoutError would be raised instead. (#59, thanks @The-Compiler for bringing up the issue and providing a test case);
• Fixed issue where the first usage of qapp fixture would return None. Thanks to @gqmelo for noticing and providing a PR;
• New qtlog now sports a context manager method, disabled (#58). Thanks @The-Compiler for the idea and testing;