Support something like "onbeforeunload" when switching tabs

[gabor-farkas]

So that the tab switching can be "cancellable".
(This doesn't actually fit well into the programming model)

[gabor-farkas]

@zkauker
Calling functions in the presenter from the renderer doesn't fit well in the programming model unfortunately.
I'd propose a solution that can bring the necessary data into the viewmodel (presenter):

• create a bindActiveTabIndex in TabSheet so that the active tab index is can be reflected in the viewmodel
• create a tabClicked(ParametricEventHandler handler) method on TabSheet. Calling this method will cancel the automatic tab-switching behaviour of the tabsheet, and the handler has to explicitly switch tabs using the bound activeTabIndex property.
• create an "automaticTabSwitch" boolean property on the model (default true), the tabClicked method will set this to false.
• create a clickedTabIndex int property on the model.
• in the tabLink.click method in the renderer put an if based on the automaticTabSwitch value. If it's false, we don't immediately set the activeTabIndex, but instead we set the clickedTabIndex.
• In the tabClicked method if the clickedTabIndex changes, we fire the given listener.

This seems like a little hack, but we need a quite fast solution for now for the business need I guess.
@zkauker if this is okay for you, then @mterjeki please do it this way.

[gabor-farkas]

it's ready