command: make 'disc-title' property writable

This commit makes 'disc-title' property writable using
STREAM_CTRL_SET_CURRENT_TITLE. This commit also contains
implementation of STREAM_CTRL_SET_CURRENT_TITLE for stream_bluray.
Currently, 'disc-title' is writable only for stream_dvdnav and
stream_bluray and stream_dvd is not supported.
stream_bluray: fix for zero-based title index for Blu-ray

The title for stream_bluray DID start from 1 and I misunderstood
that it started from 0 because mpv accepted bd://0 as a proper
argument. In fact, 0 title was an alias for the longest title but
it was not handled as a special value. This commit fixes these
behavious. 'disc-title' property for Blu-ray now starts from 0 and
the default title can be specified by 'longest' title just like
stream_dvdnav: bd://longest. Of course, 'longest' can be omitted.
manpage: mark disc-title as writeable
x11: implement window dragging by grabbing

We don't check whether the WM supports _NET_WM_MOVERESIZE_MOVE, but
if it doesn't, nothing bad happens. There might be a race condition
when pressing a button, and then moving the mouse and releasing the
button at the same time; then the WM might get the message to initiate
moving the window after the mouse button has been released, in which
case the result will probably be annoying. This could possibly be fixed
by sending _NET_WM_MOVERESIZE_CANCEL on button release, but on the
other hand, we probably won't receive a button release event in this
situation, so ignore this problem.

The dragging is initiated only when moving the mouse pointer after a
click in order to reduce annoying behavior when the user is e.g.
doubleclicking.

Closes #608.
Merge remote-tracking branch 'upstream/master'