automatically show content-assist in property editors

[martinlippert]

as a follow-up from #164 we should enable the automatic content-assist that was available in the old-generation STS3 property editors. In those editors, content-assist for properties showed up automatically after started typing without the need to hit the keyboard shortcut for content-assist. We should enable that again for the STS4 generic-editor-based property editors.

[spring-projects-issues]

(comment in Pivotal Tracker added by Kris De Volder:)

Looks like this is as simple as setting a broad set of 'trigger characters' for completions.

I made it so that we can now configure this via application.yml properties as 'languageserver.completion-trigger-characters'.