Removing unnecessary range checks for QT backend support's setTrackbarMin and setTrackbarMax fucntions

Fixes

Setting truncate option for QString leftJustified method to false fixes 10 character truncation issue noted in issue 9489

checks whether the window exists and is visible. On QT closing a window
merley hides it, so the common hack for checking whether a window exists

exists = cv2.getWindowProperty(.., 0) >= 0

does not work.

we are drawing on a BGR image declared as RGB, so we should not swap
components here either.

amends work done in pull request #6976.

QSlider does not have getMinimum/Maximum getters. Probably was not
compile tested.

Conflicts:
	modules/highgui/src/window_QT.cpp

highgui: Support to change trackbar count in setTrackbarPos and replaced deprecated CreateToolbarEx in Windows.

highgui: Support to change trackbar count in setTrackbarPos and replaced deprecated CreateToolbarEx in Windows.

The documentation states, that a NULL or an empty window name can be used
to refer to the control panel. But the string parameters of the C++ frontend
methods cannot be NULL and converting an empty string to a const char* by
c_str() doesn't produce a NULL pointer, but an empty string. Unfortunately,
the const char* pointer is just passed on to the standard C functions in
the QT backend, which doesn't check for the empty string case.

There are two places where the empty string check could have been introduced:
inside the frontend or inside the backend. As long as the documentation only
mentions this as a special case for the QT backend, the best place seems to
be there.

fix should be applyed for every window interface and generation

QVector range check exception fixed.

I don't know why it didn't work for the original author, but it definitely
works now.