When adding a new video, the whole region around the plus button should be clickable

[colinbdclark]

Currently, the layer's add video button (consisting of an SVG + icon) can only be activated with mouse if you click right on the +. Instead, we should have a transparent underlay or a more complex click handler that allows users to click anywhere within the video preview's rectangle.

[colinbdclark]

For example, if the user clicks where the mouse cursor is in the screenshot, nothing will happen. Instead, they have to hover right over the + icon.