Prefix Denite sources with something recognizable

[languitar]

Currently, the Denite sources are just called references, documentSymbol, or contextMenu. This somehow pollutes the global source namespace and is also hard to recognize. Especially the relation of contextMenu to the language server is hard to grasp from the name and contextMenu also implies much more than just language server stuff. Thus, I'd propose to prefix the source names with something like lc/ or ls/ to make them easily recognizable and to highlight their relation to language servers.