Gremlins show action now uses a z-index of 2000

Gremlins action used default z-index, which made it invisible on
Bootstrap modals for instance. Using a higher z-index make the gramlins
actions visible on most situations. The max z-index of a bootstrap page
is 1050.

Closes #68
Merge pull request #75 from marmelab/action_z_index

Gremlins show action now uses a z-index of 2000