Dialog: allow merging of css classes between global options and instance options

[rewritten]

When an application has global styles defined, the natural thing would be to configure some class through the provider, like (sorry for the CoffeeScript code, I can't write plain JS anymore....)

myModule.config ["$dialogProvider", ($dialogProvider) ->
  $dialogProvider.options
    modalClass: "some-global-class"
]

But then an instance of Dialog can't add another class, it must specify the whole list of classes:

$dialog.dialog
  modalClass: "some-global-class specific-class"
.open()

Thus, it's a burden to track everywhere the global dialog class. If one could have something like

$dialog.dialog
  modalAdditionalClass: "specific-class"
.open()

this could be added to the globally configured one.

[skytracer]

+1

Especially useful for different messageBox type, eg "errorBox", "confirmBox" etc

[pkozlowski-opensource]

It will be handled in #441