Add the visualization gui classes

Make access to controllers non-const (maybe the controller will be modified)

Merge branch 'feature/VariableModel' into develop

Adds Variable model in the Variable controller

Creates the variable model The variable model holds a vector of variables. The Variable struct will be completed afterwards

Inits variable controller and adds it to the SciQlop app

Initializes the variable inspector and adds it to the MainWindow

(Minor) Extracts ui of DataSourceWidget

Add target to copy MockPlugin library in "plugins" directory after compilation

Merge branch 'featureCompilationFixForCI' into develop