Add large paddings for SignInPage

All pages should have large paddings on the left and right
edges. Items that have interactive areas should expand to full width
(if intented to be full width) but their content should have large
paddings.
Use label property of the TextField instead of additional Text item.

TextField has implicit large paddings in left and right edges.
Move PageHeader inside Flickable

Previously PageHeader was not moving along with the
content. This problem was visible when keyboard
was open.

This also moves ScrollDecorator inside Flickable.
Fix binding loops coming from ScrollDecorator during startup

Use VerticalScrollDecorator when only vertical scrolling is
needed.
Merge pull request #83 from rainemak/sailfish-port

Polish application startup