cache: redo options and default settings

Some options change from percentages to number of kilobytes; there are
no cache options using percentages anymore.

Raise the default values. The cache is now 25000 kilobytes, although if
your connection is slow enough, the maximum is probably never reached.
(Although all the memory will still be used as seekback-cache.)

Remove the separate --audio-file-cache option, and use the cache default
settings for it.
README: describe release cycle
manpage: mention bdnav://
input: fix compilation on windows

Currently I don't have a crosscompilation toolchain, so I couldn't test
whether this actually compiles (and still can't).
input: allow disabling window dragging with --no-window-dragging

Requested in github issue #608.