Separate edit fields

Created by: ederag

This PR is first step towards verbatim entry. To fix #280 (closed) and #423 (closed) some changes will be required in db.py (working on it).

Meanwhile, using completion code/widgets that were still around, this PR brings back separate field entries, as well as completion for time, activity, category and tags. That should fix #301 (closed) and #439 (closed).

Feedback welcome !

To do:

• End time popup is too large
• Fix empty start time issues
• Find more standard icons, and/or check icons availability, provide fallback ? cf. https://github.com/projecthamster/hamster/pull/461#issuecomment-541577877
• separate "go-down-symbolic" and clear icons