selection: auto-scroll: selection keeps scrolling while mouse is outside grid

Moving the mouse outside the grid while we have an on-going selection
now starts a timer. The interval of this timer depends on the mouse’s
distance from the grid - the further away the mouse is, the shorter
interval.

On each timer timeout, we scroll one line, and update the
selection. Thus, the shorter the interval, the faster we scroll.

The timer is canceled as soon as the mouse enters the grid again, or
the selection is either canceled or finalized.

The timer FD is created and destroyed on-demand.

Most of the logic is now in selection.c. The exception is the
calculation of the timer interval, which depends on the mouse’s
position. Thus, this is done in input.c.

The scroll+selection update logic needs to know a) which direction
we’re scrolling in, and b) which *column* the selection should be
updated with.

If the mouse is outside the grid’s left or right margins, the stored
mouse column will be -1. I.e. we don’t know whether the mouse is on
the left or right side of the grid. This is why the caller, that
starts the timer, must provide this value.

The same applies to top and bottom margins, but since we already have
the scroll *direction*, which row value to use can be derived from this.

selection.h

@@ -74,3 +74,8 @@ void text_from_primary(
     struct seat *seat, struct terminal *term,
     void (*cb)(const char *data, size_t size, void *user),
     void (*dont)(void *user), void *user);
+
+void selection_start_scroll_timer(
+    struct terminal *term, int interval_ns,
+    enum selection_scroll_direction direction, int col);
+void selection_stop_scroll_timer(struct terminal *term);