Update context of mask widgets in background

Currently all changes of model are immediately visualized in view.

Widgets are not hidden in their intermediate state, which may lead to this after loading the project:

Size of widget grew in the middle, while bounding rectangle of masks still uses the deprecated size of the plot viewport.

It is desirable to hide widgets until all their sizes and contents have been set.

Also: clear content of widgets BEFORE clearing models (e.g. on creating a new project). Otherwise there is a moment when widgets and graphics refer to already deleted items ==> big chance for segfault