avidemux: rewrite derivation

This drastically reduces the complexity of the `avidemux` derivation
and adds QT5 support (see #33248).

Rather than invoking `cmake` over preconfigured hooks, it's much easier
to use the `bootStrap.bash` script provided by the developers to do the
installation tasks. Furthermore this script makes it way easier to
configure which parts of `avidemux` should be used (e.g. CLI-only) or
without the plugins.

In order to create a CLI-only instance you can simply override the
derivation:

```
avidemux.override {
  withQT = false;
}
```

It's possible to set the default executable as well (`avidemux` creates
a `avidemux_qt5` and `avidemux_cli` executable by default):

```
avidemux.override {
  default = "cli"; # default is `qt5`
}
```

The GTK support has been dropped entirely since it was originally broken
in our system and can't be built ATM. Other distros such as ArchLinux
don't support GTK anymore (see https://git.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/avidemux#n64)