Add a PaneButton to list of allowed widgets for NavigationPane

[tobiasht]

Is your feature request related to a problem?
I want to call a function when pressing a button in the NavigationPane. For example pressing Save, at the moment the button will just navigate to a new screen.

Describe the solution you'd like
Add a new type PaneButton to the list of allowed widgets for items and footerItems. When pressing this button a function will be called instead of navigating to a new page.

Describe alternatives you've considered
Add a commandBar, which can be changed based on the selectedTabIndex

[bdlukaa]

you can use the onChanged callback to call a function when a button is pressed.

onChanged: (i) {
  if (i == 0); // first button was pressed
}

PaneButton is not an existing widget

[tobiasht]

Ah that's true. I can do some additional checks there. Thanks for the advice.

PaneButton was meant as a suggestion to make it more intuitive.