add show parameters keyboard shortcut

Please allow showing the completed parameters as a tooltip, without invoking the whole completion machinery.

Attached screenshots show the limited amount of positions where pressing ctrl+space results in the function prototype showing.

IntelliJ Idea has a "show parameters" tooltip. By pressing ctrl+p in that IDE, I can show the prototype of the function where the cursor resides, while typing the argument list. In Qt Creator it's awkward and breaks concentration.