Merge QQuickLineControl into QQuickTextInput.

There's no clear separation of responsibilty between these classes and
keeping them in sync and forwarding signals is a unnecessary overhead
that can be avoided by combining them.

Task-number: QTBUG-22627
Change-Id: I4350eb3c612b10d4ed34886374889ae893b8183a
Reviewed-by: Martin Jones <martin.jones@nokia.com>