Replace Qt keywords with `Q_*` macros

[yhgu2000]

Since the words signals, slots and emit are some common names. They have potential name collisions when mix Qt code with other libraries. This problem has been reported many times:

• https://stackoverflow.com/questions/22188432/qt-macro-keywords-cause-name-collisions
• https://stackoverflow.com/questions/16638077/qt-name-collision-but-no-keywords-not-an-option
• https://stackoverflow.com/questions/37788273/c-qt-variable-named-slots-in-included-external-library

As to myself, I encountered this problem when using OpenCV, a well-known computer vision library before.

So it is recommended to disable them in new projects: https://doc.qt.io/qt-5/signalsandslots.html#using-qt-with-3rd-party-signals-and-slots .

[antonypro]

@yhgu2000: Sorry for the time time to reply!

I'm going to merge your pull request.