Fix #153 - Log / Error Notification System

Fix #153. Adding log window to "Help" menu to display log messages. Highest priority log message shown in status bar (latest one only), with an icon to indicate the nature of the message.

- [x] Add menu option to "Help" menu (opens empty window currently)
- [x] Set up log window with list and headings
- [x] Create custom log handler class to populate and add to list
- [x] Add support for filter based on priority (filter visibility only)
- [x] Custom slider for selecting filter level
- [x] Add latest high priority message to status bar
- [x] Add (custom?) icons indicating message nature
- [x] Double-clicking icon or latest log message shows log
- [x] Add calls to translate for log class
Consider `setRange(0, len(self.log_levels) - 1)`