Add support for integration tests to the UI

[stevemessick]

This does a few things:

• Change the icon for integration_test to indicate it is a test source directory
• Change the icon for integration test files to indicate the file is a test source
• Add the device id to the flutter_tools command
• Stop displaying duplicate menu items on the main() test run button in the gutter

We also need to update the flutter_tools project templates to mark integration_test as a test source root.