[bean completion proposals] improve filtering mechanism to allow continues typing

It seems like continues typing while bean completion proposals show up shows weird behavior. The proposals sometimes initially don't show up, then you continue typing, they do not show up, then you stop typing and invoke content-assist manually, they show up.

Most likely this is related to filtering (done by the client) and the prefix. If the bean proposals are not included at a specific place when the completion proposals are show up for the first time, continuously typing does not invoke the completion proposal request via the language servers again. Instead, the client filters previous results down to the additionally typed characters. At least I am guessing that this is related to what I am seeing.

Need to find some concrete situations to reproduce this. Tbd...