FilesCollection: add getUpdateSites(boolean) method

This method returns a Collection<UpdateSite>, which will end up being
ordered consistently due to the usage of LinkedListHashMap internally.
AvailableSites: add initializeAndAddSites method

This method not only parses the list of available sites from the ImageJ
wiki, but also adds them to the given FilesCollection. That way, the
FilesCollection will always contain the expected update sites.
Updater: immediately initialize & add all update sites

Previously, the sites were only discovered and added when the
SitesDialog was actually popped up. This caused the FilesCollection data
structure to be incomplete and incorrect until such a GUI element was
constructed and presented.

The new behavior properly and fully initializes the FilesCollection much
earlier, so that it can be interrogated as needed in various places.