Remove QgsAbstractProperty and subclasses, and instead use a single
QgsProperty class which covers the use of QgsAbstractProperty
and its subclasses. This simplifies the API and avoids pointer
handling. QgsProperty is implicitly shared for memory efficiency
and inexpensive copies.
Instead of defining the suitable field types and values for
properties when registering them to a data defined button,
now properties are fully defined when the valid
property keys are defined.
instead of individually setting map canvas, expression context, etc
Will make it more easy in future to add additional parameters
to the widget context
* Save more data to QML
* Virtual fields
* Map tips
* Display expression
* Read only flag
* Streamline expression context generation
Whenever an object is able to generate an expression context it
implements the method createExpressionContext() declared in
QgsExpressionContextGenerator.
This makes a cleaner API and allows using QgsFieldExpressionWidget and
QgsDataDefinedButton from python because standard OO programming
approaches are used instead of callbacks and void pointers.
* Colorize output of doc and sip tests
* Fix build
* Fix sip complaints
* Fix rebase problems
* Workaround failing bindings test
