Start to bring back text selection

You can (for) now toggle lines as selected via click, and copy
using the keyboard. SELECTION is filled automatically when the
selection changes. The text copied to the clipboard is a clip-
board-friendly serialization rather than 1:1 what's in the view.

Next:

• Bring back the within-hovered-message selection mode from the earlier prototype
• Range selection by press-and-move w/ autoscroll
• Item view-like keyboard selection
• Don't copy pre-formatted HTML in the clipboard serialization
• Hook up the context menus again, which include copy actions