Minor improvements

This pull request adds a chat message when a user uses /inbox clear so the player receives feedback that the command was successful. 

This adds a chat message when the player uses /inbox and they do not have any messages, so they receive feedback that the command was successful.

This delays the onjoin mail notification chat message, it seems sending chat messages or formspecs in the registered onjoin function aren't received by the client, but delaying those actions seems to make things work as expected.

Added chatplus._players[key].lastcount variable, this keeps track of the message count the last time the hud was redrawn, and only redraws the hud if the message count changes. At times the hud would flicker, only redrawing the hud when the message count changes fixes that, and this will reduce any network traffic (as little as it may be) caused by redrawing the hud every 5 seconds.