input: make input queue thread safe

If pthreads are enabled the input queue accesses are regulated by acquiring
a mutex. This is useful for platforms like OS X, where the events are created
in the cocoa thread and added to the queue to then be dequeued in the playloop
thread.
macosx: remove platform specific input queue

Since last commit the input queue in the core is thread safe, so there is no
need for all this platform specific stuff anymore.
cocoa_common: add precise scrolling support

This adds precise scrolling support. I ran some tests and it seems a little
bit smoother and well.. precise. The defaults are rebindable using: AXIS_UP,
AXIS_DOWN, AXIS_LEFT and AXIS_RIGHT.
input.conf: bind AXIS_(LEFT|RIGHT) to seek 5

Binding them to volume was a bad default, so change it.
changes.rst: document some OSX and windows changes