a "Preview" view to preview grammar while editing

[mickaelistria]

In order to make it more efficient to edit textmate grammars, it would be great to have a "Preview" view (or maybe to populate the Outline view while we don't have anything better to show there). That would allow user to put in any text, and to see the text highlighting change while the user edits the textmate grammar file.

[angelozerr]

@mickaelistria you can reuse https://github.com/eclipse/tm4e/blob/master/org.eclipse.tm4e.ui/src/main/java/org/eclipse/tm4e/ui/internal/preferences/TMViewer.java which is used for preview in the preferences

[angelozerr]

But before using TMViewer please wait for my commit at #94 because I will do some refactoring.