If there is an available archived zip with the version of the plugin
currently installed, we can use it to cancel/abort the upgrade of the
plugin. This is internally handled as the installation of the archived
zip and goes through all the validation and confirmation.

Additionally, some other parts were improved. Most notably, renderer no
longer decides itself if some installation can be cancelled but it
always asks the controller (plugin manager).

The button for installation was moved to the left so there should be
first buttons to add things, and then buttons to cancel things (which is
common in normal forms).