Check if App is already deployed

Currently an app is deployed if it is selected. We should consider to add an option, where the deployment process checks if the app on the Confluence server is already up-to-date.

We need to decide how to treat SNAPSHOTs. Either there is a force parameter or we can detect from the app's metadata provided by Confluence if the local app is newer.

For App Projects we should force the deployment of SNAPSHOTs by default.