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

bin/app/data/gui/tabbed_view/tabbed_view.css

@@ -121,3 +121,10 @@ QTableView QTableCornerButton::section {
 	border-bottom: 1px solid <color:table/table>;
 	border-top-left-radius: 10px;
 }
+
+#help_button {
+	background: transparent;
+	border: none;
+	/*margin: none;*/
+	/*padding: none;*/
+}