Enhance the PluginDeployer API

[dhuebner]

Feature Description:

When testing extension installation from file, I noticed, that when installing an already installed extension a "Failed to install" error is reported. This is because local file deployer rejects installing existing extensions, but because the installing routine can only check the number of installed extension (in this case it's 0) a misleading error is reported.
I think we need to change the installation routine return value to something more meaningful than just a number. With more information we can then produce a better user feedback. See PluginDeployerImpl.deployPlugins().

In addition it would be nice to have a possibility to be notified when a plugin was installed or uninstalled to properly update the VSXExtensionsWidget.

See also: #13291