fix(widgets) Widgets should be reactive to prop changes

[chrisgervang]

For #7457

Background

Found non-reactive props while working on #9314. Widgets should be able to change their placement or view as the user changes these props. It's just up to the widgets to actually implement how this is done..

Change List

• Internal changes to widgets to update members as props change.

[coveralls]

coverage: 91.703%. remained the same
when pulling ea973f5 on chr/reactive-widgets
into 05f8c95 on master.