Multiple selection of morphs

[kathwill]

As a presentation creator, I want to be able to select multiple morphs to change the features of several morphs more efficiently

Acceptance criteria:

• [ ] When pressing a button (e.g. shift), it is possible to select more than one morph
• [ ] When you click on a morph that is already selected, it will be unselected
• [ ] After selecting several morphs and changing a feature, they should change accordingly
• [ ] When clicking somewhere else, all selected morphs get unselected

Estimated effort: 13