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
