Improve the export performance.

[daithiocrualaoich]

Currently there is some noticeable UI freezing during Favicon export. Some options for improving this include:

• Do ico resizing in the background as user is picking the output location.
• Do png conversion in the background as user is picking the favicon sizes.
• Separate thread in Cocoa for resize.
• Use a progress bar dialog to reduce the perception of UI freezing.