Extract compose_state.js.

This is mostly just moving methods out of compose.js.

The variable `is_composing_message`, which isn't a boolean, has
been renamed to `message_type`, and there are new functions
set_message_type() and get_message_type() that wrap it.

This commit removes some shims related to the global variable
`compose_state`; now, `compose_state` is a typical global
variable with a 1:1 relationship with the module by the same
name.

The new module has 100% line coverage, most of it coming
via the tests on compose_actions.js.  (The methods here are
super simple, so it's a good thing that the tests are somewhat
integrated with a higher layer.)

static/js/compose_state.js

@@ -0,0 +1,57 @@
+var compose_state = (function () {
+
+var exports = {};
+
+var message_type = false; // 'stream', 'private', or false-y
+
+exports.set_message_type = function (msg_type) {
+    message_type = msg_type;
+};
+
+exports.get_message_type = function () {
+    return message_type;
+};
+
+exports.composing = function () {
+    // For legacy reasons, this is the same as get_message_type.
+    // Most callers use this in a boolean context, but there are
+    // some stragglers that inspect the string value.
+    //
+    // TODO: Fix callers who care about stream/private to use
+    //       get_message_type(), and then convert this to return
+    //       `!!message_type` or something like that.
+    return message_type;
+};
+
+function get_or_set(fieldname, keep_leading_whitespace) {
+    // We can't hoist the assignment of 'elem' out of this lambda,
+    // because the DOM element might not exist yet when get_or_set
+    // is called.
+    return function (newval) {
+        var elem = $('#'+fieldname);
+        var oldval = elem.val();
+        if (newval !== undefined) {
+            elem.val(newval);
+        }
+        return keep_leading_whitespace ? util.rtrim(oldval) : $.trim(oldval);
+    };
+}
+
+// TODO: Break out setters and getter into their own functions.
+exports.stream_name     = get_or_set('stream');
+exports.subject         = get_or_set('subject');
+// We can't trim leading whitespace in `new_message_content` because
+// of the indented syntax for multi-line code blocks.
+exports.message_content = get_or_set('new_message_content', true);
+exports.recipient       = get_or_set('private_message_recipient');
+
+exports.has_message_content = function () {
+    return exports.message_content() !== "";
+};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = compose_state;
+}