Use only QtPy abstraction + HelpDev for checks, so we can reduce the code, many imports and try's that checks the binding before insert ideas of PR #135.

We can still use the same API, e.g, load_stylesheet_pyqt5(), load_stylesheet_from_environment(), load_stylesheet(pyside=False) here introducing the idea of PR#135 for auto-change the binding.

Still there, remove deprecation message

load_stylsheet_pyqt4()
load_stylsheet_pyqt5()
load_stylsheet_pyside()
load_stylsheet_pyside2()

Add new API

load_stylesheet(qt_api='')

Deprecation warning: Prefer load_stylesheet('pyside2')

load_stylesheet(pyside=True)

Deprecation warning: Prefer load_stylesheet(os.environ['PYQTGRAPH_QT_LIB'])

load_stylsheet_from_environment(pyqtgraph=True)
They still using old and new API until the release of v3. I've already pushed this in branch develop.

We can add a new function to load_stylesheet using the new version with qtpy with another name, for example, just stylesheet(), get_stylesheet() or we can keep the same name, however, we cannot have both at the same time.

Also, we can reduce the number of compiled things - see the case of 'example' in develop version.

Add known-issues in the docs.