Added a get_items_from_selection for the Keys TreeView

We probably will use this functionality as long as we use a TreeView.
This makes accessing the information a bit easier.
KeysPage: Added a "key-selected" signal

This shall facilitate that the application can be split up more easily.
Using this signal for receiving the selected key hopefully make
writing different GUIs a bit easier than integrating with the
(unrelated) parts of the application.
KeysPage: Emitting key-selected signal when hitting a (non-existant) button

We will need to trigger that callback somehow as there is no
implementing button just yet.
KeysPage: Made keySection an optional argument

Its only purpose is to disable a button higher up the widget stack.
We can use signals now. This is the preparation for getting rid of this
intertwined connection.
Sections: Connected to the key-selected signal instead of passing a reference

This looses the coupling a little which is good.
KeysPage: Emitting key-selected when the selection has changed
KeysPage: Removing optional keySection argument

We do not need that any more as we emit a signal.
Added an example program which uses the KeysPage widget
KeysPage: Introduced a new key-selection-changed signal

This is to distinguish the commitment of a user to a key from mere
browsing the list of available keys.

I'm not quite sure whether this is actually needed, so there is a good
change that this will be deleted again.
KeysPage: Connected the simple GUI to the new signal
KeysPage: Reworked the UI into an HPane

A new attempt for the GUI: We show some details of the key when the user
selects it and offer a button to advance the wizard.  This is actually
an attempt to get rid of the wizard.
Sections: Reacting on the key-selected signal by advancing the wizard
KeysPage: Added documentation for the class
Moved KeysPage class into the KeysPage.py file
KeyPresentPage: Accept a fingerprint during initialisation

This helps to have a more reusable widget to display the fingerprint and
a QR-encoded version of it.
KeyPresent: Added a simple demo program to present a key

This is still a bit rough. From an architectural perpesctive, we want to
have a generic secure copy function which utilises that widget.
However, we're not quite there yet, so we're only refactoring bits and
pieces for now.
Sections: Using the KeyPresentPage from the KeyPresent module
KeySignSection: Reworked the UI to make the wizard buttons work

This is to maintain compatibility with the old buttons.  The intention
is to remove them though.

This also got rid of the middle page, leaving two pages only: The first
to select a key and the second with the details for the secure transfer.
KeySignSection: Removed the next button

This simplifies the UI by removing a rendundant button.
KeySignSection: Removed the Back button

and only show it when we actually can go back.
For now, it has been placed on the top. A bit like a HeaderBar. But that
may not be the nicest. Feel free to shuffle it around until it is more
pleasent.