Daily statistics

There should be a Statistics button that opens a new window. That window should contain date selector and a list of daily activities for that day from the log. Current date should be selected by default.

Whether currently tracked activity is shown there or not should be dependent on whether it was already saved to the log file or not (see also #4 — that's the reason currently tracked activity may be periodically saved to the log file).

Be careful with race conditions when writing / reading a file simultaneously.