Hide tray icon at exit via ctrl-C

[HebaruSan]

Problems

• On Windows, the tray icon doesn't disappear at close if you kill CKAN with ctrl-C in the console. (You need to use the --show-console parameter to keep it visible after the GUI appears.) You can even accumulate several of them if you do it over and over:
  
• If you use the tray icon to open the settings while CKAN doesn't have focus, it appears at the upper left corner of your monitor
• Tray icons have the ability to display tooltips, but we don't have one

Changes

• Now the Console.CancelKeyPress ctrl-C event handler hides the tray icon
• Now we always pass a reference to the main form when we show the settings dialog, which will ensure it is properly centered on the main window
• Now the tray icon has a tooltip that says "CKAN"

[techman83]

LGTM, this is something I've had to do in other applications