ui: Explain differences of errors and how to fix them (issue #501)

* added help button next to filters in error table
* added menu action Help -> Fixing Errors
* added help button on indexing dialog
* show help message after first time indexing with errors, if not seen already

src/lib/utility/messaging/type/MessageShowErrorHelpMessage.h

@@ -0,0 +1,23 @@
+#ifndef MESSAGE_SHOW_ERROR_HELP_MESSAGE_H
+#define MESSAGE_SHOW_ERROR_HELP_MESSAGE_H
+
+#include "utility/messaging/Message.h"
+
+class MessageShowErrorHelpMessage:
+	public Message<MessageShowErrorHelpMessage>
+{
+public:
+	MessageShowErrorHelpMessage(bool force = false)
+		: force(force)
+	{
+	}
+
+	static const std::string getStaticType()
+	{
+		return "MessageShowErrorHelpMessage";
+	}
+
+	const bool force;
+};
+
+#endif // MESSAGE_SHOW_ERROR_HELP_MESSAGE_H