[FIX] web_calendar: error in case of creation with quick_create disabled

Broken in 82fee7dfe8 porting of quick create to Bootstrap modals: in
case quick_create is disabled, the creation of the quick_create dialog
is bypassed and so is defining this.$input, but focus() calls still
happen and blow up.

Create a dummy $input in the constructor so it's alway possible to call
this.$input.focus().

addons/web_calendar/static/src/js/web_calendar.js

@@ -967,6 +967,7 @@ openerp.web_calendar = function(instance) {
 
             // Can hold data pre-set from where you clicked on agenda
             this.data_template = data_template || {};
+            this.$input = $();
         },
         get_title: function () {
             var parent = this.getParent();