[xml] provide content-assist for constructor-arg name

[martinlippert]

follow-up from spring-attic/spring-ide#420 (comment):

please add content-assist for: <constructor-arg name="...">

[martinlippert]

Fixed with f185db5

[martinlippert]

@jpraet if you want to try this, you can grab a CI build from here: http://dist.springsource.com/snapshot/STS4/nightly-distributions.html