etc: add an input config file that reverts recent key binding changes

This is for users who don't like changes. I'm hoping it will make the
process of cleaning up key bindings less bumpy.

It should be mentioned in the release notes of the next release.

etc/restore-old-bindings.conf

@@ -0,0 +1,31 @@
+
+# This file contains all bindings that were removed after a certain release.
+# If you want MPlayer bindings, use mplayer-input.conf
+
+# Pick the bindings you want back and add them to your own input.conf. Append
+# this file to your input.conf if you want them all back:
+#
+#    cat restore-old-bindings.conf >> ~/.config/mpv/input.conf
+#
+# Older installations use ~/.mpv/input.conf instead.
+
+# changed in mpv 0.6.0
+
+ESC quit
+
+# changed in mpv 0.5.0
+
+PGUP seek 600
+PGDWN seek -600
+RIGHT seek  10
+LEFT  seek -10
++ add audio-delay 0.100
+- add audio-delay -0.100
+( add balance -0.1
+) add balance 0.1
+F cycle sub-forced-only
+TAB cycle program
+A cycle angle
+U stop
+o osd
+I show_text "${filename}"