(js) Improve sg-search directive

Will now respect the "listRequiresDot" source parameter and uses
ng-messages to show an error if the minimum number of characters is not
reached.

Fixes #438, #3464

UI/SOGoUI/UIxComponent.h

@@ -86,6 +86,10 @@
 /* SoUser */
 - (NSString *) shortUserNameForDisplay;
 
+/* Common defaults and settings */
+- (int) minimumSearchLength;
+- (NSString *) minimumSearchLengthLabel;
+
 /* labels */
 - (NSString *) labelForKey:(NSString *)_key;
 - (NSString *) commonLabelForKey:(NSString *)_key;