Make [button:N] mean the invoking button, as retail does

[button:N] was a held-state modifier: IsMouseButtonDown(name), true while you
physically hold that button. Retail's is a property of the activation -- which
button clicked the action button -- and a keybind press counts as button 1
because it activates through the left-click path. Different in kind, and the
visible break was [button:1] Rejuvenation; Regrowth casting Regrowth from a
keybind, where retail casts Rejuvenation.

It now reads ClassicAPI's GetMouseButtonClicked, defaulting to LeftButton when
no click dispatch is running. That default IS the keybind rule: Button:Click()
with no argument reports LeftButton, and a macro run from anywhere else answers
button 1 the same way.

This also reverts 891467c. That commit refreshed the action bar on
GLOBAL_MOUSE_DOWN/UP so the icon could track the held state -- under the real
semantics there is no such state to track, and the icon correctly sits on
whatever button 1 resolves to. PLAYER_TOTEM_UPDATE stays; it shared the comment
and inherits it.

Requires ClassicAPI v1.15.8, up from v1.15.0. v1.15.8 scoped
GetMouseButtonClicked to the click dispatch; before it a HELD button kept the
value alive (by design -- OnDragStart reads it), so turning the camera with
right-click down made every keybind press read RightButton. The function exists
below v1.15.8 and answers wrongly, and a conditional that is quietly wrong is
worse than one that warns, so the floor moved rather than the conditional being
gated.

Bare [button] / [nobutton] survive as an extension retail has no equivalent for,
now meaning "a click dispatch is running" -- the practical "activated by a
keybind, not a click" test, and no longer leaning on anything but the documented
nil.

ClassicAPI.lua

@@ -3,7 +3,8 @@
 
     ClassicAPI is a client mod (sibling to Nampower/SuperWoW) that backports the
     modern C_* API into the 1.12.1 Lua environment. It is a HARD REQUIREMENT of
-    this addon (ClassicAPI v1.15.0+, which added frame:RegisterUnitEvent), so the
+    this addon (ClassicAPI v1.15.8+, which scoped GetMouseButtonClicked to the
+    click dispatch; v1.15.0 added frame:RegisterUnitEvent), so the
     wrappers below call the API directly — no fallbacks. The load-time requirement
     check (Core.lua) uses IsAvailable() to warn when the DLL is missing and
     HasMinimumVersion() when it's too old; users who don't want ClassicAPI should