Scatter/Marimekko: Allow to dismiss selected elements by clicking into a dead area

[sophiamersmann]

#3381

• If chart elements are directly selectable by clicking on them, then it should be possible to deselect elements by clicking into dead space
• Currently, the only way to deselect elements is to go into the entity selector and then click on "Clear"
• This is currently relevant for Scatter plots and Marimekko charts (slope charts already support 'clicking anywhere to deselect entities' behaviour)