Improve the call editor.

[Lattyware]

Currently the call editor has some usability and responsiveness issues, and doesn't support editing certain things (e.g: #9).

[Lattyware]

A good suggestion for an improvement from Lattyware/massivedecks#167 (comment):

... I think there should be some easy way to see the capitalization in the editing screen—e.g. a semi-transparent text abc in the slot