Click first cell child on space keyup

[yuriy-fix]

Fixes vaadin/vaadin-confirm-dialog-flow#120

Important note: Changes the grid behavior by clicking the cell on Space keyup. However cell is still "activated" on Space keydown.

[pekam]

I think this makes a lot of sense, considering that buttons normally get triggered on key up.

I just wonder if active-item changes should follow the same pattern...

EDIT:
One weird behavior caused by using keydown for changing active-item: When you keep holding the space key down, the active-item keeps toggling between null and the focused item (because keydown keeps on firing when the key is being held down).

[yuriy-fix]

I was thinking about that, but explicitly preserved the activeItem behavior in order to avoid breaking changes to existing applications.

Active-item behavior is working this way on master and probably somebody relies on it.
I guess we can discuss changing it and create another issue if needed.

[pekam]

Yeah, I'm also a bit afraid to make this kind of changes...

But I do think that the case is pretty much the same: To trigger an action, whether that is through a button's click listener or active-item-changed, it should happen consistently on key up.

Thinking about the bug ticket, it might be also a valid use case that you want to open a confirm dialog for some action when "activating" the item anywhere in the row, instead of only via the button in one column. But if you'd open the confirm dialog on active-item-changed event, you would stumble upon this bug again.

I also think that if changing the active-item-changed to trigger on keydown is considered a breaking change, then the same is true for this click behavior. It's just maybe not as risky.

But I'm not sure about my personal opinion, whether this should be changed now or not...

@tomivirkki, I would like to hear your opinion on this, or even more preferably the absolute truth. ;)

[pekam]

LGTM

A separate issue will be created for considering the case of activeItem.