Feature/decorators settings

this MR adds persitence for decorators; it uses a [canvas] group in the ini file of the application to store all the parameters of the decorators (visibility, color map, etc.)

ex with sw2d:

[canvas]
Cells_colormap=inferno
Cells_field_name=alphak
Scalar%20ColorMap_colormap=density
Scalar%20ColorMap_field_name=water_depth
Scalar%20ColorMap_show_scalarbar=true
Scalar%20ColorMap_visibility=true
Scalar%20Glyphs_colormap=inferno
Scalar%20Glyphs_field_name=surface_elevation
Scalar%20Glyphs_visibility=false
Scalar%20Isolines_colormap=inferno
Scalar%20Isolines_field_name=surface_elevation
Scalar%20Isolines_isolines_count=25
Scalar%20Isolines_visibility=false
Vector%20Streamlines%202D_colormap=PuBu
Vector%20Streamlines%202D_field_name=unit_vector_discharge
Vector%20Streamlines%202D_show_scalarbar=true
Vector%20Streamlines%202D_visibility=true