SitesDialog: dump exception when activating a site

Previously it was impossible to know *why* an update site was considered
to have an invalid URL, because we caught Exception and ate it.
MediaWikiClient: move to core imagej.util package

This is a generally useful class that need not be Updater-specific.
MediaWikiClient: add default ctor w/ FIJI_WIKI_URL

This makes it more convenient to construct a MediaWikiClient
specifically for use with the ImageJ/Fiji wiki.

This change also adds a TODO to remove this Fiji-specific reference from
the ImageJ2 codebase. It might work to simply use wiki.imagej.net as is,
but I didn't test it, so I left it as a TODO for now.
Split out Swing-agnostic update sites logic

It now lives in its own class, AvailableSites, in the updater core.

This class could still use lots of work (e.g., all methods are still
static right now), but it is a start.
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.
Merge remote-tracking branch 'origin/fix-update-sites'

This branch performs not only some clean-ups with regard to where classes
belong between ij-updater-core and ij-ui-swing-updater, but also fixes a
bug reported by Olivier Burri: suggested update sites could no longer be
activated with a single click on the check box.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>