Implement a picker for major events, using the new editable string picker.

It's used not only in special node editing (for Event Happens and If Event Happened?)
but also in the Townperson Advanced, Talk Node, and Quest editors.

doc/editor/Specials.html

@@ -336,6 +336,7 @@ the response changes (the first field) to 90, and the Event which prevents the c
 second field) to 4. If Fred was killed (i.e. if the Goblin Chief was not killed before Day
 90), she will respond with the second text field. Otherwise, she will respond with the
 first.</p>
+<p>You can also use events in quests. An quest can be set to expire if an event hasn't happened by a certain day. This is more or less the same as making dialogue vary.</p>
 <p>Finally, you can use the If-Then Special Node Type 150: Special Thing happened?, which
 calls different special nodes depending on whether a day has been reached and whether an
 Event has happened before that day or not.</p>

doc/editor/appendix/Specials.html

@@ -252,7 +252,7 @@ occurred. See <a href="../Specials.html#time">Passage of Time</a> in the chapter
 explanation.
 <dl>
 <dt>Mess1, Mess2:</dt><dd>Standard usage.</dd>
-<dt>Extra 1a:</dt><dd>The number of the special event. (Range 1 ... 20)</dd>
+<dt>Extra 1a:</dt><dd>The number of the special event.</dd>
 <dt>Example:</dt><dd>If the slaying of the goblin king is event 4, when it is killed, call
 this special node with Extra 1a set to 4.</dd></dd>
 
@@ -1310,7 +1310,7 @@ reached before a given event happened (see the chapter on the passage of time fo
 explanation).
 <dl>
 <dt>Extra 1a:</dt><dd>The day some special thing is supposed to happen.</dd>
-<dt>Extra 1b:</dt><dd>The number of an event (0 .. 20). This is the event that would
+<dt>Extra 1b:</dt><dd>The number of an event. This is the event that would
 prevent the special thing from happening (leave at 0 for no event). If the day in 1a has
 been reached, and this special event did not occur before the day in 1a, then the special
 in Extra 2b is called.</dd>