Make player control bar inherit pointer-event styling. This way the 
Editor can set it to none, while acting normal in the Player.

#524

Also restructure files containing interfaces.

#386

- Use scss extension fpr styles file

This way images can't be drag-and-dropped into droplist elements.
Also avoids confusion about what is draggable on a page.

This is necessary to avoid playing parts of audios and videos twice
when a unit is reloaded, since audios and videos send their playback
time only every 5 seconds when they are played back.

- Remove prefix 'on'

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.