launcher: Enable scrolling for the GridView to display more apps

launcher: Enable scrolling for the GridView to display more apps

This enables the GridView to allow scrolling and implicitly be able to
display more than a dozen of applications. The interactive mode controls
whether this is enabled or not, if the items is higher than what the grid
can contain (which is 4x3 in our current item cell dimension). If
interactive is enabled we use vertical scrolling, with some commented
out examples on how to enable horizontal scrolling. In tests,
horizontal scrolling caused some animation artefacts while vertical
seemed to behave much more reliable.

With it, we simplify the QML considerably by removing IconItem. Most of
the functionality has been retained, being copy-pasted from IconItem
like displaying icons, or using the first letter of the application name
in case there's no icon.

Note that this change removes The icon movement entirely, which seemed
to cause quite a bit of issues while draging/dropping scrolling in the
the grid itself. This seems to remove one feature for another, but by
just enabling interactive resulted in a jerky inconsistent animation.

Bug-AGL: SPEC-3028

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I64810dbd7ef014ed0473fd92df67807d10cc36ba