fixed html tags
show background logo and gradient in welcome screen

Note: Because QTextDocument lacks some css support, the logo cannot be centered. Instead I'm using Qt stylesheets to achieve that. The outcommented code shows how it should be done.
added WebKit Qt module
use QWebView instead of QTextBrowser for the welcome screen
cleanup of welcome html
adjusted css to webkit and moved the background image to bottom right
Support Qt5 deps
Merge pull request #75 from macgitver/docs/WelcomeScreen

Background in Welcome screen