set_active_input_by_source: do not check for oldpad == None

Going from ready to playing can make it be None. Besides, there's
nothing wrong with that.
theme loading now wroks
adds master out level monitor widget
shows master level monitor on main ui.
do not go on stopping writers if we are not recording.
core: correctly (re) set the sync property on xvimagesinks

    this time for real, I promise...
core: replaced input-selector with a videomixer

todo: animate properties instead of plainly switching
core: (fix?) initialization

WHY, WHY do we need this? After replacing the input-selector with an videomixer
the whole app upon starting behaved slow as a snail, BUT, the same pipeline when
run with gst-launch-1.0 was fine with more or less the same amount of cpu load.

Trial, error and the mighty stick revealed that doing a
[PLAYING->]READY->PLAYING state change fixed everything.
basic crossfade between cameras
only show state changes for main pipeline
also expose the main preview
adds generic image source
core: support to add a default background source
load a basic background

just so I don't forget it
core: handle disconnection of multiple devices at the same time

    now it really really works. word.
    Still it'd be better if we put some fake source if there are no inputs left.
factor out transitions
VideoMixerTransition: cleanup, logging. stacking order
transitions: adds configurable duration
VideoMixerTransition: adds horizontal slide transition
transitions: use correct scaling for properties

With many thanks to Sebastian Droge, now the range [0..1] is correctly
mapped for xpos and ypos.