extremely slow scrolling between days on manta

Scrolling is fast on the desktop, so this may be isolated to Manta, or may be isolated to Mir

It's also slow on N4 and can be reproduced the following way:

* have two locations
* scroll some days into the future in the first location
* switch to the second tab
* switch back to the first tab
* now it scrolls slowly back to the current day

It's not noticeable on desktop, so it will be related to mir or qt5.2 perhaps.

App running with performance overlay

It's a bit slow on my N4, but not nearly as bad as the Manta videos, and the top performance graph rarely goes above 10ms while performing the steps Martin describes above

Setting the highlightMoveDuration explicitly to 400 (the default) for ListView fixes at least the slow scrolling on the nexus4. Can't test it on manta, though. Seems like there's an underlying bug with this property.

If someone could test the related branch on manta would be cool!

Further investigation:

On desktop the default value for "ListView.highlightMoveDuration" is 150, where on the device the default highlightMoveDuration is -1 (means it will take as much time as necessary).

So I've updated the branch with highlightMoveDuration explicitly set to 150

Fix committed into lp:ubuntu-weather-app at revision 231, scheduled for release in ubuntu-weather-app, milestone alpha-1