Some small icon improvements

* 128 × 128 px icon for FileZilla package
* Changed iconUrl for FileZilla to Wikimedia Commons.
* Added icon for Ketarin package
* Replaced Skype icon with the current official desktop icon (128 × 128 px)
* Corrected iconUrl of VirtualBox package, because currently it’s wrong (icon not displayed in the Gallery https://chocolatey.org/packages/virtualbox)