Various fixes, see description

- removed obsolete entries in QRC
- added new banner_empty.png for a more sailfish like view
- changed opacity to have have the look I wanted
- tried to have similar banner size on UserPage and MeTab
- replaced search icon to fit the style of other tab icons
- fixed inbox.svg
Fix logout
Reworked about page

- Added credits from Openrepos description
- renewed DicksonBetaDB avatar, as old one was low-res
- added avatar placeholder image

That placeholde image could be also used while avatars of users are
loading (or error) in TweetDelegate.
Fix comments
Merge pull request #88 from Morpog/sailfish-port

Various fixes, see description