ZSTD-4699 deployment.xml editor does not apply the dark theme

The editor is refactored to embed the WTP XML editor in the
deployment.xml tab and the Properties File editor in the
deployment.properties tab. This way the syntax coloring of the dark
theme is applied correctly. As a bonus the depoyment.xml tab now have
code completion, validation, code folding and other benefits inherited
from the WTP XML editor.