Improvements to icons

[marticliment]

Improvements to how the icon cache works

• Improve mimetype detection for URLs without extension filename

  • This fixes .ico files not rendering on Widgets for UniGetUI
  • This allows other-than-png files to be added to the icons and screenshots database
  • This eases handling of different image formats

• Improvements to the TaskRecycler

  • Fix a race condition that would result with a second task being created when another one was already running but hadn't been added to the dictionary

• Resize icons for a smaller impact on disk

  • Certain icon formats and sources will be downsized to 256x256