tweet-info-page: Fix the source label's margin_right

to align up with the follow button etc.
list-list-entry: Use GtkGrid

Let's flatten the hierarchy a bit
ListListEntry: Use GtkStack instead of menu structure

this is cleaner and helps with the coming GMenuModel transition.
ListStatusesPage: Make the description a GtkEntry

A maximum of 110 characters doesn't really justify the use of a
GtkTextView, plus this saves us the trouble of a GtkScrolledWindow.
ListStatusesPage: Don't crash when going forward

... in which case list_id will be 0 and the subsequent call to args.arg
will segfault.