Users miss automated support of input: tooltip

Related to #12

Tooltip

1. User types "="
2. Any additional character, example "Su", typed triggers a tooltip that displays functions starting with that prefix
3. User types the an additional "m", tooltip changes displaying now further helpful information on all the accepted parameters of that particular function

This already happens on LibreOffice, it also happens on MSO.

opendesk-12-lo-calc