rewrite tab creation

Each tab is an instance of the new Tab-module. The share, downloads and
info tabs were rewritten to use it.
They are all added to the tab registry in each player.

Known Issues: The chapter tab is disabled until it is rewritten as well.
rewrite chapter tab

The chapter tab is an instance of the Tab-module as well. It is added
to the tab-registry.
Each chapter tab gets a reference to the update function to enable
highlighting of the currently played chapter.

TODO:  There is a need for a global update hook that can be listened to
by each tab.