Add chanopt for notifications

[shouya]

Hexchat has the functionality of showing an OSD notification with libnotify, but it's only available in the global settings dialog. Not as other three alerting methods (beeping, blinking tray icon, etc), as far OSD notification isn't able to set for specific channel as 'extra alert' (Right-click the channel -> extra alert -> OSD notification).

OSD notification is named 'show tray balloons' in the settings. I can set it globally here.

I really hope there could be a same option here just as the other three notification methods for me to enable/disable the OSD notification for a specified channel.

$ hexchat --version
HexChat 2.10.2

I'm on gentoo linux with xmonad.