Add content assist for available functions

[fsteeg]

Like content assist for Flux commands (see metafacture/metafacture-flux#4, ui & impl), we should implement a corresponding functionality for collectors and functions (see https://github.com/metafacture/metafacture-core/tree/master/metamorph/src/main/resources).

It might actually make sense to implement this before the Flux commands assist, since we need to read only a single file (1 for collectors, 1 for functions) instead of gathering all flux-commands.properties from each Metafacture module.

[dr0i]

The XSD has more infos , e.g. boundaries and descriptions, so it may be good to use this as source.
In our daily scrum the idea came up if we should use annotations in java classes (as for the flux to generate the flux-commans.md). (IMO we should stick to one to avoid the danger of loosing sync of documentation.)

[TobiasNx]

Since this ticket is talking about collectors and function, is it related to the interims fix? Can it be closed? @fsteeg

[fsteeg]

Still relevant for functions, changed title.

[TobiasNx]

@fsteeg is this a ticket for the Extension?

[fsteeg]

It would be great both in the extension and the playground.