Mark some older XFCE software in our tree as deprecated.

These are all components abandoned upstream, and not developed
anymore.

All of these still depend on GTK2 and some on the already deprecated
x11-toolkits/libxfce4gui.

As you may know XFCE development has already dropped any support
for GTK2, so, unless these software packages are ported to GTK3 by
someone, they WILL be removed from the tree when XFCE 4.16 will be
released and ported.

Due to their status these ports could break for many reasons and
unless the fix is really trivial or patches are provided they will
be removed from the tree in case of breakage.

Further reasons for each of them follow.

xfce4-mixer:

Removed from XFCE in version 4.12, abandoned since, in 2015.
Uses outdated gstreamer 0.10.

xfce4-print:

Removed from XFCE after version 4.6, abandoned since, around 2009.
CUPS support broken since cups version 2.3.3 due to upstream interface
changes.
Already deprecated due to dependency on libxfce4gui.

xfce4-quicklauncher-plugin:

Unsupported after XFCE 4.4, around 2009.
Already deprecated due to dependency on libxfce4gui.
Unable to locate a source code repository.

xfce4-wmdock-plugin:

Last update in 2013.
Unable to locate a source code repository.

There are other components I'm not marking as DEPRECATED right now,
but please note I have doubts on their support status. These are
also still depending on GTK2 and it's unsure if they will be ported
to GTK3 and spared by the XFCE 4.16 update:

orage
xfce4-embed-plugin
xfce4-mailwatch-plugin
xfce4-equake-plugin

Other XFCE components in our tree still depending on GTK2 are
reasonably expected to be ported by upstream to GTK3 before 4.16
is released, or, in some cases have already been ported in their
repositories and/or development versions.