Introduce mouse release and mouse drag events

Introduce separate mouse release and mouse drag (move while pressed)
events: MouseLeftRelease, MouseLeftDrag, MouseRightRelease etc,
to allow binding them to actions independently from mouse press events
(MouseLeft, MouseRight etc).

This change:

- Makes it possible to handle mouse release and drag for arbitrary mouse
  events and actions (including Lua actions), not just for MouseLeft as
  in the current code.

- Fixes issue #2599 with PastePrimary and MouseMultiCursor actions:
  selection is pasted not only when pressing MouseMiddle but also when
  moving mouse with MouseMiddle pressed; similarly, a new multicursor is
  added not only when pressing Ctrl-MouseLeft but also when moving mouse
  with Ctrl-MouseLeft pressed.

My initial approach was not to introduce new events for mouse release
and mouse drag but to pass "mouse released" info to action functions
in addition to *tcell.EventMouse to let the action functions do the
necessary checks (similarly to what MousePress is already doing). But
then I realized it was a bad idea, since we still want to be able also
to bind mouse events to regular key actions (such as PastePrimary)
which don't care about mouse event info.

runtime/help/keybindings.md

@@ -407,8 +407,14 @@ mouse actions)
 
 ```
 MouseLeft
+MouseLeftDrag
+MouseLeftRelease
 MouseMiddle
+MouseMiddleDrag
+MouseMiddleRelease
 MouseRight
+MouseRightDrag
+MouseRightRelease
 MouseWheelUp
 MouseWheelDown
 MouseWheelLeft
@@ -520,11 +526,13 @@ conventions for text editing defaults.
     "Esc": "Escape",
 
     // Mouse bindings
-    "MouseWheelUp":   "ScrollUp",
-    "MouseWheelDown": "ScrollDown",
-    "MouseLeft":      "MousePress",
-    "MouseMiddle":    "PastePrimary",
-    "Ctrl-MouseLeft": "MouseMultiCursor",
+    "MouseWheelUp":     "ScrollUp",
+    "MouseWheelDown":   "ScrollDown",
+    "MouseLeft":        "MousePress",
+    "MouseLeftDrag":    "MouseDrag",
+    "MouseLeftRelease": "MouseRelease",
+    "MouseMiddle":      "PastePrimary",
+    "Ctrl-MouseLeft":   "MouseMultiCursor",
 
     "Alt-n":        "SpawnMultiCursor",
     "AltShiftUp":   "SpawnMultiCursorUp",
@@ -629,10 +637,12 @@ are given below:
         "Esc": "AbortCommand",
 
         // Mouse bindings
-        "MouseWheelUp":   "HistoryUp",
-        "MouseWheelDown": "HistoryDown",
-        "MouseLeft":      "MousePress",
-        "MouseMiddle":    "PastePrimary"
+        "MouseWheelUp":     "HistoryUp",
+        "MouseWheelDown":   "HistoryDown",
+        "MouseLeft":        "MousePress",
+        "MouseLeftDrag":    "MouseDrag",
+        "MouseLeftRelease": "MouseRelease",
+        "MouseMiddle":      "PastePrimary"
     }
 }
 ```