Add option to hide the daily activity summary

[3-w-c]

Currently, after working on a few different activities in a day, the "daily summary" expands in width, pushing the start of the activity field off the screen.

This PR adds a dconf-only option (no UI yet) to hide the activity completely, to work around this issue.

A better fix might be to make the summary flow onto multiple lines; I went with an option to hide it instead because it seemed easier – and I'm fine launching hamster if I want to see the summary.

[mwilck]

I haven't observed this problem, even though I usually have quite a few activities during the day.
Can you post a screen shot?

[3-w-c]

Here's a shot of partway through the day, showing the popup already getting pretty wide. I'll post another one if I get enough different activities today to push it all the way off the screen.

[mwilck]

I'm getting it, thanks. I don't have so many different Categories, therefore I'm not observing it myself.