Updater (Swing): reinstate the call to checkWritable()

Somewhere in the migration to ImageJ, the Fiji updater lost that
check whether we can write into the user's directory to begin with.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: add a helper to detect if we're running inside C:\Program Files

We had so many problems with Windows' security model interfering with the
ImageJ updater trying to update files inside C:\Program Files. It is time
to do something about it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
UpToDate check: do not bother checking inside C:\Program Files

Most likely, we'll have problems to update files there anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing): trigger an error when running inside C:\Program Files

This addresses http://trac.imagej.net/ticket/1371.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'updater-windows'

Most notably, this addresses http://trac.imagej.net/ticket/1371 -- a big
fat warning when ImageJ/Fiji was installed into C:\Program Files on
Windows Vista or later.

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