Provide an swt-headless fragment

[laeubi]

There are several cases where one want to run some code in an headless way that might has references to UI code.

AWT offers for this purpose the java.awt.headless system property.

Something similar would be good for SWT:

1. Have a static method Display.isHeadless() returning false for all current implementations
2. Have a swt-headless fragment, where all public API is simply a no-op as far as possible or throwing an SWT exception and isHeadless return true.