fixes #878

fixes #975

That way we can autohide the scollbar if needed, it should fix some
jumping issues, it makes it possible to flick on mobile, etc.

Some related bugs:

https://bugreports.qt.io/browse/QTBUG-75223
https://bugreports.qt.io/browse/QTBUG-44902

fixes #903

- This ensures that gstreamer bus operations work
on macOS and windows, such as enumerating devices.

Allows you to zoom and pan now.

relates to #647

This adds a man page which lists command line options, key bindings and
commands. A new CMake option `MAN` is added, enabled by default on
`UNIX` but not `APPLE`.

The man pages are generated with either `a2x` (from asciidoc) or
`asciidoctor`. If both are not found, a fatal error is signaled.

See also:
https://stackoverflow.com/questions/1394910/how-to-tame-the-windows-headers-useful-defines