spacing
allow installation of handlers based on element-keywords

this is a BAD filter...
proxy the eventkeyhandlers function
callback for displaying the audio-levels
make 'closeable'test a subtest of self.streamer
register self.streamcontrol's callback for setting audio-levels

LATER: is this thread safe? (gst-events calling qt-methods)
added level-meter
using floats in config-file
mak previewWidget resizeable
keep aspect ratio of previewWidget on resize

Closes https://github.com/iem-projects/striem/issues/18