[2.2] enable completion as you type

Enables completion "as you type" instead of double-tab. Use tab to move through the completions-list. Thus creates a faster and more intuitive possiblity to run programs.

Things that might need to be changed:
- Move through completions-list using arrow-up or arrow-down (like in Cinnamon-menu). Arrows are currently used for history. Anyways I'm not sure if a history is needed in runDialog.
- Add completion on pressing backspace. Might look weird though.

In general I think runDialog should behave more like Cinnamon-menu than like a console.