Update libs/widgets to c++11

Summary:
Use clang-tidy to:

• Use default wenn possible for constructor and destructor
• Use nullptr instead of 0 or NULL
• Use for (type &element : list) when already using std iterator (.begin() .end() )

if this patch is acepted, if will do the same for the entire calligra codebase.

Subscribers: Calligra-Devel-list

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D19132