#21 ER: Add option to log chats to files

[Christoph Daniel Schulze]

Add the following features:

* Enable / disable logging of the current chat / server
panel by clicking a menu item (Channel->Log and
Server->Log respectively) or using the context menu.

* Add the following options to the config dialog:

1. Automatically save server logs, channel logs and
private chat logs.

2. Specify a location to keep the logs in.

The log files are saved in subfolders named after the
network they belong to. The log files are named after
the server address, the channel name or the user name,
with the extension ".log".