gui: use XML-based page for splash screen instead of a static jpg

For devices with an unusual aspect ratio, stretching a bitmap to
the screen resolution is sub-optimal. So let's reuse our XML engine
that allows assembling the splash screen from individual parts.

The splash page is loaded from a separate XML file for quick loading.

Change-Id: I2d3dad26e42fcefaf563dacdf0ffa61f209dada1