Fix issue 180

Closes #180 (closed)

@tobias.bernard The placeholder overlay now in two modes, 'welcome screen' and 'empty-state placeholder'. There are 6 content variations depending on the availability of SMS and IM accounts. The welcome screen appears only at the first startup, while the empty-state overlay shows up as soon as the chats-list got cleared.

One can bring the welcome screen back by resetting the flag in ~/.purple/prefs.xml as follows:

<pref name='first_start' type='bool' value='1'/>

Here are a couple of example screenshots: