Explicitly remove the webapp tree during windows installation

It is possible that if people put files into webapp (or adjust them), then they will survive an upgrade.

The non windows install explicitly does this, but the windows one relies on the installer.

We need to force the installer to do this.

rough plan

1) Search to set a property if [INSTALLDIR]\webapp exists. Run before REMOVEFILES
2) Custom action to set a property to [INSTALLDIR]\webapp
3) Component which specifies
<util:RemoveFolderEx On="both" Property="propertyFrom[2]"/>
and which is conditionalized on the propertyfrom[1]

Note that (1) may be impossible and/or we could use the same property in both cases.