Adds optional cancelButton to end of tabs

[megoth]

By providing option onClose, tabs are extended with an extra, unstyled tab that contains a cancelButton. Once clicked it will call the given onClose-method.

This is needed for providing the cancelButton for the global dashboard.

Part of fix for SolidOS/solid-panes#154

[timbl]

Please test with updated version of t .../tests/

[megoth]

Rebased onto master, and tested with the test-files.

I did check that the original tests went through, but the test-setup is waaaaaay to slow that I want to use it to test this new part as well.

[megoth]

@timbl should I add a new tests, or is it ok to just check the existing tests?

[timbl]

The existing tests build a tabs widget and morph its state, with each orientation. The idea is you can for example check the positioning CSS works. I suggest just adding the option for the X box to the test. Add a dummy onClose function to the core of the test loop. (Didn't I do that?) so that you can see that the positioning CSS works for that too in each orientation. I don't think you need to check that it works without the X button again.
Sorry I thought that was clear

[timbl]

Did you run the tests on #110 with the X button included?
onClose() included?
Did the x pup up in a reasonable place for each orientation?
If so I am fine to merge

[megoth]

Ok, I've made it kinda work - but we need to have a discussion on the tests; They are not easy to read, and since there are no documentation I have to go a lot back and forth between the HTML, Turtle and JavaScript to make sense of it. I'm not a fan of these kinds of custom test setups, and we should investigate the possibility of making use of snapshots, like Snapshot Testing in Jest.