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/settings/ApplicationSettings.cpp

@@ -506,6 +506,16 @@ void ApplicationSettings::setUpdateVersion(const Version& version)
 	}
 }
 
+bool ApplicationSettings::getSeenErrorHelpMessage() const
+{
+	return getValue<bool>("user/seen_error_help_message", false);
+}
+
+void ApplicationSettings::setSeenErrorHelpMessage(bool seen)
+{
+	setValue<bool>("user/seen_error_help_message", seen);
+}
+
 int ApplicationSettings::getPluginPort() const
 {
 	return getValue<int>("network/plugin_port", 6666);