Allow users to enable automatic project configuration

[fbricon]

When modifying a pom.xml or a gradle file, java project configuration and classpath should be updated. While it's possible do trigger a manual update (ctrl+alt+u or cmd+alt+u), it'd be more convenient to do it automatically, as requested by @pmuir.

The proposed change is to display a warning when a build file has been modified, asking the user to update the java configuration. Choices are

• Never: no project configuration will be triggered and the message won't show up again
• Now: project configuration will be triggered and the message will show up next time
• Always: project configuration will be triggered from now on and and the message won't show up again

[pmuir]

@fbricon my other comment was that I wouldn't necessarily realise that "Update project configuration" would be what would cause the project classpath to update (unless I had used Eclipse).

[fbricon]

@pmuir that should be clearer, hopefully: