ShinyApps UI integration

This change adds UI support for the ShinyApps package, if it's installed. The following commands are supported via a new ShinyApps menu under Tools when a Shiny file is open: 

- *Deploy App*: Also available on global toolbar. Shows existing deployments of current directory, and allows the user to re-deploy over any existing deployment or create a new deployment. Largely UI sugar to form a `deployApp` command.

- *Terminate App*: Terminates the application deployed from the current directory, if one is running. When we support Configure I expect this will look more like a task-manager (i.e. a full list of all apps from all accounts with configure/terminate invokable on any app); for now it relies on context. 

- *Manage Accounts*: Always available. Connects and/or disconnects accounts.

For now I've re-used an icon for deploy, but we should consider making a new one for this and possibly the other commands (esp. if ShinyApps is going to get logo/branding assets).