Add keyboard shortcuts to context menu

[nevries]

Expected Behavior

When a context menu is shown, I can choose an item by clicking on it OR pressing a keyboard-shortcut.

Actual Behavior

When a context menu is shown, I can choose an item ONLY by clicking on it.

This would improve modelling speed. Especially when you are zoomed out, its sometimes hard to hit the elements with mouse cursor. Have a look at the attached proposal, maybe we can start with shortcuts for the first menu level and see how it works out before thinking about do it for second level also (with second level I mean the menu that shows up after pressing the wrench button).

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Related to: #314

[philippfromme]

We generally want to be able to display tool tips including the keyboard shortcuts, not only for the context pad.

[hkupitz]

Since I've had a user ask me about this today, I'd like to bring this topic up again. I agree with @nevries that having keyboard shortcuts for placing/adding new elements would be greatly beneficial for power users to speed up the modeling process.

I could not only imagine this for the context menu actions of selected elements, but also in general to attach a new element of a given type at the mouse cursor, ready to be placed onto the canvas.

Here is an overview of already existing hotkeys as well as potential new ones for the other elements of the toolbar: