Big proxy fix (addressing hopefully most complaints on mailing list and
in tracker)
* proxy render settings are now independent of render size settings.
That means: which proxy size is used, is controlled by two parameters
now: the proxy size of the files, can be controlled with additional
buttons within the proxy panels. What is shown in a specific preview
window depends on the header settings of the preview panel.
So: proxies are _only_ used in those windows, that are switched to a
specific proxy resolution.
* output rendering is always done _without_ proxies.
* proxy generation now shows a waitcursor with numbers.
(closing Bug: [#18909] Building Proxies doesn't give any feedback
which was rather a feature request, since the problem mentioned there
was always the case :) )