This prevents the section from being placed in front of elements
with negative z-index

Better logical order with featrures first and bugfixes last.

This reverts commit 87534c53.

The entire HTML is no longer saved, but only the indices and the color
of the markers

- Use the same default state for interactiveMuteControl
and for playerProps.interactiveMuteControl

- Ignore blank pages when calculating the response progress

- Use the same font properties for input element and button element
- Change cross out behaviour. The button now behaves as
toggle button and sets the focus to the input element only when it is
crossed through.

If the virtual keyboard with numbers (or with numbers and operators) is
used, input via the normal keyboard is also restricted to these
characters.