* Small refactor for the widgets of Cutter
This refactor include the following :
* Creation of a new class
Creation of a new class, named CutterWidget, that inherits from QDockWidget and
is used to represent all of the widgets of the main window.
The goal of this class is to regroup all the behaviour shared by the widgets of
Cutter.
For example : in the constructor, instructions corresponding of those
present in the macro **ADD_DOCK** (in MainWindow.cpp) are executed.
This was made because I think that the macro **ADD_DOCK** which is used
to construct the widgets does not take advantage of the object structure.
* Ensure that every widget has a parent
Some widgets were created using the constructor QDockWidget, but using
**nullptr** (default) as argument, thus they haven't got any parent.
The constructor of a CutterWidget takes as argument the MainWindow and an
action (optional) and calls the constructor of QDockWidget with the main
window as argument. This is valid under the assumption that it is mandatory
for every widget to have the main window as a parent.
* Constructors removal
The constructors of some widgets are not used anywhere and does not seem not
fullfill any current usecase. They were removed.
* Renaming CutterWidget to CutterDockWidget
* Remove QML QtWebEngine dependency
* Remove Web graphs from Dashboard
* Remove web graphs from SidebarWidget
* Remove web mini graph from PreviewWidget
* Remove all remaining html and QtWebEngine dependencies
* Update docs/Compiling-with-CMake.md after QtWebEngine removal
* Remove QtWebEngine from Travis
