Fix relevant information in API documentation

The API documentation for QML types and C++ classes are inconsistent and in many cases, lack the information important to the user.

For any given type or class (for example, Rectangle):
A) Qt version it was introduced in
B) module/library name to import("QtQuick 2.0")
C) Parent type or class
D) Types or class inheriting from it
E) If the class is instantiated as a QML type or the C++ class a type instantiates
F) The QT variable for enabling the module
G) #include directive (C++)

For A, B, E, and F, there needs to be QDoc context commands. QDoc can deduce C, D, and G.

Variables:
A) \since (same as current \since command)
B) \inqmlmodule and \qmlmodule (same as current implementation)
E) \instantiates (same as current \instantiates command)
F) \qtvariable <variable>, where variable is from http://doc-snapshot.qt-project.org/qt5-stable/qmake/qmake-variable-reference.html#qt

Suggested wording:
A) "Introduced in: Qt <version>"
B) "Module: <module name>"
E) "Instantiated by: <QML type>", "Instantiates: <C++ Class>"
F) "Add to .pro file: \c

"