Add interface for adding custom tab context menu actions

[DamirPorobic]

We could provide an interface for adding a list of actions which we could show in the tab context menu. Every action could be expecting a single parameter, the index of tab over which it was clicked.

[DamirPorobic]

@nicolasfella Hey Nicolas, I have this feature that I want to implement in the annotator. As you probably know, we support now tabs and there are some operations that can be done per tab by opening the context menu. I would like to provide an option for the parent application to provide a list of custom actions that can be show in every tab, when the users clicks such an action in the context menu, I would emit the triggered signal with a single parameter, the tab id. Usually that would be simple, overwrite QAction and create a custom signal that takes the id as parameter. What's puzzling me is that kimageAnnotator is a library, which makes it a bit more complex. Do you have any idea who to set this custom class up?

[DamirPorobic]

Done