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.h

@@ -141,6 +141,9 @@ public:
 	Version getUpdateVersion() const;
 	void setUpdateVersion(const Version& version);
 
+	bool getSeenErrorHelpMessage() const;
+	void setSeenErrorHelpMessage(bool seen);
+
 	// network
 	int getPluginPort() const;
 	void setPluginPort(const int pluginPort);