sailfin: Improve layout on landscape and tablet screens

I've dropped the whole `<constant> * Theme.pixelRatio`-approach[^1]
for determining when the UI should split into two columns, because
the values seemed quite arbitrary and I was entering random numbers.
I'm now doing it on multiples of `Theme.itemSizeHuge`, which is easier
to reason about.

This also fixes occasions where items in a grid would leave a bit of
space to the right in the CollectionPage.

Backdrop images in VideoPage and MusicAlbumPage now have a maximum
height of half of the screen, to avoid filling the entire screen in
landscape mode. Perhaps it doesn't always look good, but it makes the
layout more usable.

Images on the SeasonPage and MusicAlbumPage (in landscape) are now
aligned to the right, to avoid blocking the Page back indicator.

sailfish/qml/pages/ConnectingPage.qml

@@ -2,6 +2,8 @@ import QtQuick 2.0
 import Sailfish.Silica 1.0
 
 Page {
+    allowedOrientations: Orientation.All
+
     PageBusyIndicator {
         running: true
     }