There are a couple methods in GTKNativeStyle that are called repeatedly, such as
getXThickness(), getYThickness(), and getFontForState(). These methods always
call down to native (and synchronize on the GTK lock object) even though the answer
will be the same everytime for that style object, which is a lot of wasted effort.
We should be able to grab these values once and cache them when the style object is
constructed. GTKNativeStyle objects appear to be recreated when the GTK theme is
changed, so there should be no harm in caching these values in this manner.