For big files with many differences, the compare view freezed eclipse,
if the line spacing was set. The reason were the necessary resources to
calculate the spacing in the method getHeightBetweenLines. We have to
save the heights in a list and use them to increase the performance.
Clear line heights cache at property changes.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2143

Signed-off-by: Denis Ungemach <denis.ungemach@sap.com>