Read-only presentation of values that require special treatment in run configurations

https://github.com/GoogleCloudPlatform/google-cloud-eclipse/pull/2568#discussion_r150060393

Having fields for certain special values in a run config is a real pain. We've experienced many subtle and annoying bugs not to mention the huge implementation complexity, leading to fragile behaviors.

For example, the account selector is for saving/loading an email, but the email cannot be shown if it's not logged in. What is worse is that the run config framework assumes what is present in the UI is/should always in sync with the value saved in the run config. That is, if you want to preserve the stored email while the account selector is not selecting any due to the user not logged in, you need some trick to break that sync assumption.

Another example is the project selector. In addition to the logout problem, it takes a long time to fetch projects. However, doing things asynchronously in a run conflig has always been very fragile, especially when switching between run configs, which reuses the current tab instead of creating a fresh new tab.