checkbox-gui:qml:ManualInteractionDialog.qml: Restore the scrollable description

"activeFocusOnPress: false"  as preventing the Flickable sub component to
work properly.

Call "instructions.focus = true" on Dialog completion to allow mouse scroll
without the need to first click on the TextArea.

Also removed the text comment and the useless Text element.
"automatic merge of lp:~sylvain-pineau/checkbox/fix-1362112/ by tarmac [r=sylvain-pineau,zkrynicki][bug=1362112][author=sylvain-pineau]"