QmlProfiler: Handle enable and show signals for zoomslider in C++

As it's hard to pass arguments for signals from non-QML-mapped objects
to QML objects handle the signal in C++ instead and just directly set
the properties.

Task-number: QTCREATORBUG-10943
Change-Id: I039f6938db3d7e64ca1a4bcff2f0f6aa79c65219
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
WelcomeScreen: Scroll the example grid view with the main scroll view

By tying the scroll offsets and visible areas of the two scrollable
elements together we avoid excessive loading times for the examples.

Change-Id: I6522ef3e6c0454e5bb05bef143953d8d1850ffdc
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
WelcomeScreen: Don't use clipping

Aparrently clipping is slow in QML, so we use elide and manually control
visibility instead.

Change-Id: Idb743a8daec04b028f103a075a5416729f0e2a16
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>