Fix/crash gui

Crash at algorithm launch on CEA Ubuntu Virtual Machine closes #36 (closed)

The crash comes from the use of QTextEditProcessingFeedBack without storing it inside calling class.

Introduction of AlgorithmWidget class that will store the common attributes for all algorithm widget :

• _feedback QTextEditProcessingFeedBack : needed for feedback display in log widget
• _task QgsProcessingAlgRunnerTask : needed for task cancellation