Move interactive updater tests into src/test/

No need to ship that code in the final artifact.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a test class for easy interactive testing

This is not a unit test; it uses the test infrastructure to build up
a working directory with two update sites and a couple of changes ,
then fires up the GUI.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: disallow mixing upload/download actions

Many users got confused about uploading and downloading. Due
to technical reasons, there is a huge difference between
installing/uninstalling and uploading to/removing from update sites.

Let's make it easier on users and just not offer to mix both.

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