Fix notification icons.

Android design guidelines say notifications should be 24x24dp, so use
that size instead of 16x16dp. Images were generated by down-scaling the
xxxhdpi ic_notification image.
Use clear_light drawable for action button.

Android design guidelines say notification icons should be all white,
which by extension also applies to action button icons. Obey to that.
Move a few hardcoded strings to string resources.
Cancel mention notification when going back to main activity.

After all, the user now can see where he was mentioned.
Improve mention/query notification.

Leverage BigTextStyle and InboxStyle and show the message content (if
available).
Make 'xx servers connected' notification a plural.
If only connected to a single server, adapt action title accordingly.
Don't notify of new PMs twice.

For a new PM, we get both a NewPrivateMessageEvent (for the channel
initiation) and a QueryMessageWorldEvent (for the actual message). Don't
confuse the user by telling him he has been queried twice when in fact
it's only once.
Fix displayed user name for outgoing PMs.

Closes #90.
Don't notify of closed PM channels.
Fix typo.