Jörg, I can sponsor this for you since I have been maintaining
shotwell in Ubuntu this year.
1. Please add a Breaks to shotwell instead of just a Replaces. It
wouldn't hurt to bump the version too like this:
Breaks: shotwell-common (<< 0.26.3-1)
Replaces: shotwell-common (<< 0.26.3-1)
I think you can remove this line though
Breaks: shotwell (<< 0.26.2-1)
2. Please remove all the obsolete patches from your debian/patches/
instead of just disabling them.
3. I'm attaching a patch to fix the install of appstream metadata.
You'll also need to modify debian/shotwell.install to install the
metadata.
--- debian/shotwell.install 2017-09-22 17:26:18.000000000 -0400
+++ ../debian/shotwell.install 2017-10-22 15:59:36.667571371 -0400
@@ -1,3 +1,4 @@
usr/bin
usr/lib
usr/share/applications
+usr/share/appdata