copy OverlayPreferenceStore from org.eclipse.ui.internal.editors.text since it is package-private)
adapted org.eclipse.jdt.internal.ui.preferences.ScrolledPageContent
option to disable foreground/background colour
categorise syntax classes
empty base Isabelle preferences page
syntax colouring preference page (issue #31)

Adapted syntax colours preference page from Scala-IDE. Currently no previewer available
better layout
include Platform preference store
set preference store on the editor (reuse the one from configuration)
react to syntax highlighting preference change in TheoryEditor
fixed property changes not propagating - need a new ScopedPreferenceStore for writing
american spelling
use IEclipsePreferences instead of IPreferenceStore for defaults
using more Eclipse-like colour scheme for Isabelle
actually using IPreferenceStore in the end (for reusability)
setup listeners to react to UI selections
a button to select Isabelle/jEdit syntax colours
some more tuning of Eclipse-like Isabelle syntax scheme
listen to syntax colouring preference changes (issue #37)
better name