Add an inline preview for threads

[helje5]

A request by email: One thing I’d like to see in Shrugs: an option to show threads in the main timeline, rather than in a popover window. I’m thinking something like a one-level hierarchical outline:

Sounds useful to quickly check threads contents, though it raises a few questions:

• should probably be limited to the last N (5?) messages, probably awkward to use for those 100 message threads
• would probably need an inline reply mechanism (i.e. some way to summon a reply textfield)

[helje5]

Issue #16 is a related issue.