Fix cartesian hover toggle

[Sizurka]

The hover toggle button couldn't be enabled on cartesian plots because the later setup completely overwrote the initial allowed groups. So just make it add the cartesian buttons instead of replacing the groups completely.

[archmoj]

@Sizurka Thanks very much for the PR.
Could you please provide a minimal example of illustrating the bug e.g. by forking this codepen?

[Sizurka]

See here. Note that there is no hover toggle button on the mode bar, despite it being enabled in the layout. Other buttons (e.x. "togglespikelines") work correctly.

[archmoj]

Looking good to me.

Please add a test somewhere here:

plotly.js/test/jasmine/tests/modebar_test.js

Line 158 in 042742d

describe('creates a custom button', function() {

Also I appreciate if you draft 5879_fix.md as described here:
https://github.com/plotly/plotly.js/blob/master/.github/PULL_REQUEST_TEMPLATE.md

Thank you!