ring,stream: Refactor the interface between the ring and the stream

Refactor and simplify the interface between ring and vte-stream; this is
required for the forthcoming rewrapping feature. Paging is now a concept
private to vte-stream. Instead of telling when to turn page, the ring now
advances the tail of the streams, and vte-stream does paging on its own
based on this information.

https://bugzilla.gnome.org/show_bug.cgi?id=336238#c74
ring: Fix stream reset

Change suspicios code: resetting row_stream to 0 is fishy, I really do think
it needs to be set to ring->writable.

https://bugzilla.gnome.org/show_bug.cgi?id=336238#c75
widget,emulation: Rewrap the lines when the window is resized

https://bugzilla.gnome.org/show_bug.cgi?id=336238
emulation: Avoid lines filled up with spaces in the restricted scrolling area

https://bugzilla.gnome.org/show_bug.cgi?id=336238#c88