[qtmozembed] Don't resume backgrounded web view

The patch fixes the bug when html5 video gets resumed even if the corresponding webview is background:

"1) Opened Browser and a YouTube tab
2) Started to play the video, after a while opened a new tab (Google search), and the YouTube video stopped playing ( which is expected)
3) Swiped the Browser app and it was backgrounded. 
4) Opened the Messages application and replied to a message 
5) Right after I closed the Messages application, the YouTube music started to play again
6) I tapped on Browser cover, opened it and checked that still Google search tab is the active tab in Browser"
Alright. Then there is a bug in how mActive is handled. Currently the QML code 

> QuickMozView {active: false} 

doesn't work as expected - all views are created active and activation happens in MozContentSGNode constructor.