window: Some re-factoring and improvements

We now try to select another contact when the active one is deleted.
window: The active contact and request can be NULL

Be prepared for that and show the welcome widget for this case.
welcome widget: Don't show a button

For consistency I removed the button to add contacts, there should only
be one button in the application to do this, and the + button should be
obvious enough.
tox: Fix function signature
contact: Fix function signatures

The strings should be const.
Add /myid command

Because of word-char wrapping in the chat view, it wraps not at all
chars though, so this looks pretty silly.