Keyboard navigation and mouse hover styling is wonky

[colinbdclark]

There are several interrelated issues related to keyboard navigation and mouse styling that need to be fixed:

• When the Composition Panel is hidden, it's contents continue to be selectable and activatable because the panel is hidden with opacity: 0;. The contents of the panel (though not the panel itself, because its show/hide button needs to remain visible) should be set to display: none; so that they are taken out of the tab order.
• The add video button no longer shows focusing and hover styling to indicate that it can be activated
• The video's border no longer show a highlight when the add button is selected or hovered over
• The add layer button no longer shows hover styling