Don't add "Copy to clipboard" item if filters menu is empty.
MainFrm: Cosmetics for stats.

- Always show bitrate statistic in avg/cur style for consistency.
MainFrm: Cosmetics for timers.

- Move DVD timing related things to TIMER_STREAMPOSPOLLER for better
  reliability.
- Save current file position if it differs more than 30s than previous.
  In case when mpc-hc is not properly closed.
Fix possible freeze during DVD playback on XP after
f2b6ade8e8185fdc5908ed88df31e68a106786a9

Refs #4299
Do not try to set channel from WebServer thread.

Fixes #4415