checkbox-touch:components: Fix Text wrapping in Flickable elements

Remove "contentWidth: childrenRect.width" as it makes the Flickable width
larger than the screen and and prevents the text to wrap correctly.
checkbox-touch:components: Remove unused ProgressBox
checkbox-touch:ProgressBox.qml: Simplify the component to be just a bar

With a predefined width/height.
"implicitHeight: units.gu(0.3)" renders as a progress line instead of a bar
checkbox-touch:main.qml: Move the progress bar in the page header

The dedicated qml Item progressHeader is used to replace the default page
header.

To use it on the page requiring the progress bar, just assign this Item to
the special page property: __customHeaderContents.
"automatic merge of lp:~sylvain-pineau/checkbox/header_progressbar/ by tarmac [r=kissiel][bug=][author=sylvain-pineau]"