Returns true if this node currently has the application-wide input focus.
A FocusNode has the primary focus when the node is focused in its
nearest ancestor FocusScopeNode and hasFocus is true for all its
ancestor nodes, but none of its descendants.
This is different from hasFocus in that hasFocus is true if the node
is anywhere in the focus chain, but here the node has to be at the end of
the chain to return true.
A node that returns true for hasPrimaryFocus will be the first node to
receive key events through its onKey handler.
This object notifies its listeners whenever this value changes.