* fix(filmstrip-only): vertically align center the toolbar

Use top 50% to position the toolbar's top at the vertical center
of the iframe. Then use transform 50% to move the toolbar itself
up 50% so its middle matches the middle of the iframe.

* squash: toolbox should center with filmstrip

Audio only mode can be used to save bandwidth. In this mode local video is muted
and last N is set to 0, thus disabling all remote video.

When this mode is enabled avatars are shown.

All z-indexes found in css files have been moved into css
variables. If the z-index is used only once, the variable
name will be the same as the selector it is used in. If
the z-index is used multiple times, then the plain name
of $zindex# was used. This allowed a more confident
moving down of the toolbar so that the new modal dialog,
with z-index 500, could display on top of it.

#1436