Add a proper API for the new header tabs & actions

[Alexandre Abreu]

could it be possible to add an "actions" api? ... something like:

Action {
readonly property string id
property bool enabled
property string icon
void triggered(Function callback)
}

Actions {
ActionList getActionsForTab(string tabId)
}

it would be nice to also add a:

Tabs {
void tabChanged(Tab oldTab, Tab newTab);
}

so that e.g. people could work on the associated actions when a switch happens