Long labels in code_prettify/2to3

[jfbercher]

And others derived from kernel_exec_on_cells.

Remove the label from the add_buttons_group and add it back "by hand" to enable the bubble help.
Fixes #1243 (shift-clik works for me) and last comment in #1102.

[jcb91]

Thanks @jfbercher. Is the space trick really necessary? label can't be undefined, or an empty string?

[jfbercher]

@jcb91. It seems necessary. Unfortunately, both label: '' and label: undefined fail with kernel_exec_on_cell.js?v=20180320174230:344 [code_prettify] error loading: TypeError: Cannot read property 'help' of undefined at toolbar.js:97. I haven't investigated what happens in toolbar.js

[jcb91]

Ok, good enough for me :)