DOCS/crosscompile-mingw: update

- Adds description of and uses $JOBS envvar in MXE instructions
- Adds MXE_TARGETS to command line instead of echoing it to settings.mk
- Prettify and sentence usage
DOCS/crosscompile-mingw: update disk space stats
DOCS/crosscompile-mingw: use settings.mk instead of make variables

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
lua: remove redundant inline documentation

Nobody will loom at this, and the proper documentation of these
functions is in lua.rst.
lua: allow giving fallback values in get_property() calls

E.g. ``mp.get_property("foo", "value")`` will return ``value`` if the
property can't be read.
keycodes: add const to a function argument
client API: add a client message event

This comes with a "script_message" input command, which sends these
messages. Used by the following commits.
input: don't let builtin bindings overwrite user bindings

This was already done, except when the bindings were in different input
sections. This commit fixed it. Useful for scripts, probably.
input: debug output for registered bindings
lua: add mechanism for script provided key bindings

There was already an undocumented mechanism provided by
mp.set_key_bindings and other functions, but this was relatively
verbose, and also weird. It was mainly to make the OSC happy (including
being efficient and supporting weird corner cases), while the new
functions try to be a bit simpler.

This also provides a way to let users rebind script-provided commands.

(This mechanism is less efficient, because it's O(n^2) for n added key
bindings, but it shouldn't matter.)
w32_common: don't set small icon

Windows will automatically choose the correct icon size if this field is
unset.
client API: add events for video and audio reconfig