v0.6.0 mouse: every wheel direction and side button named; tui: a scrollbar the pointer drives

* mouse: the wheel's four directions and the side buttons

parseSGRMouse read a report's low two bits alone, so a wheel tilted left or
right (66, 67) reported as WheelDown, and a mouse's back and forward buttons
(128, 129) as a left or middle press. It now decodes by bank: bits 0-1 number
a button within the main three and none, the wheel's four directions (bit 6),
or buttons 8 to 11 (bit 7). WheelLeft and WheelRight join MouseButton after
Forward, so every existing value keeps its number; Back and Forward are
reported for buttons 8 and 9; buttons 10 and 11, and a report with both bank
bits set, are dropped as KeyNone. A wheel step is a press with no release in
every direction.

* tui: ScrollThumb and ScrollDrag

ScrollThumb is the thumb ScrollBarStyled draws, its first row and height,
exported so an application hit-tests the bar it drew. ScrollDrag drives a bar
by the pointer, rows counted from the track's top: a press on the thumb takes
hold where it lands, a press on the track moves the thumb's middle there and
takes hold, and each drag keeps the held row under the pointer, clamped at the
track's ends, at the nearest offset when there are fewer offsets than rows.
Each call returns the offset to show.

A table of SGR reports covers each bank and drops the buttons past 9; a
property test over every track of 3 to 10 rows and content up to 30 checks
each press and drag against the thumb ScrollThumb draws.

README.md

@@ -226,9 +226,11 @@ division approximation; `SoftLight` uses init-time LUTs replacing `math.Sqrt`.
   `Key == KeyRune` with `Rune` set for printable input, `Modifiers` bitmask
   (`ModShift`, `ModAlt`, `ModCtrl`), `Action` (`KeyPress`, and `KeyRepeat` and
   `KeyRelease` while releases are reported; see below)
-- `EventMouse` — 0-indexed `MouseX/Y`, `MouseBtn` (buttons, wheel),
-  `MouseAction` (press/release/move/drag), modifiers. Enable via
-  `SetMouseMode(MouseModeClick | MouseModeDrag)`; SGR protocol only.
+- `EventMouse` — 0-indexed `MouseX/Y`, `MouseBtn` (the three buttons, the
+  wheel's four directions, back and forward), `MouseAction`
+  (press/release/move/drag; a wheel step is a press with no release),
+  modifiers. Enable via `SetMouseMode(MouseModeClick | MouseModeDrag)`; SGR
+  protocol only.
 - `EventResize` — new `Width`/`Height`
 - `EventPaste` — `Text` holds a whole paste, as sent, control characters
   included. Enable via `SetPasteMode(true)` (bracketed paste); off, a paste