Add ability to edit entries

[imiric]

The current t backend SQLite UI is not user-friendly, and editing entries using SQL is troublesome and error-prone.

This would have a similar UI to #1, i.e. an interactive environment where you could alter the description, duration, date of entries, as well as split or join them.

Some of it would also be possible directly on the command-line (e.g. "t split 1 '20-80'" would split entry 1 into 2 entries, 20% and 80% of the duration of the original entry).

For joining entries, a new entry would be created which would be a "parent" of the joined entries. This way any time calculation done on the parent would involve the children. The children could possibly be hidden from the frontend, to avoid confusion.