command: don't append, but prepend deinterlace filter by default

In most cases, it's better if deinterlacing happens before any other
filtering, so prepend the filter to the user's filter list, instead
of appending it.
m_property: rearrange code
m_property: add a way to switch on property values in property expansion

Allows for example: --status-msg='${?pause==yes:(Paused) } ...' to
emulate the normal terminal status line. It's useful in other situations
too.

I'm a bit worried about extending this mini-DSL, and sure hope nobody
will implement a generic formula evaluator at some point in the future.
But for now we're probably safe.
command: when changing a property, allow showing an extra OSD message

This is for properties that normally show a bar, and thus do not show an
OSD message (as per classic mplayer behavior). Setting an extra_msg
allows showing an OSD message anyway, except if OSD messages are
explicitly suppressed.

This refactors the whole show_property_osd() function a bit, and
replaces the weird sep field with a more general method.
command: when changing volume while muted, show an extra message on OSD

So nobody has to wonder why everything is silent, even if they raise the
volume.