Prevent context drawer from disappearing

Currently, on plasma mobile, if you navigate from a page that contains contextual actions to a page without contextual actions and back, context drawer disappears. Looking for contextual actions in pageStack.layers.currentItem only when pageStack.layers.depth > 1, then checking pageStack.currentItem and finally, checking pageStack.lastVisibleItem, we ensure that we look for contextual actions in the most relevant page.

Differential Revision: https://phabricator.kde.org/D19977