cleanup: Move inkscape.appdata.xml.in out of root directory

[Bryce Harrington]

The file inkscape.appdata.xml.in exists in Inkscape's source code root directory, but doesn't need to be there and should be moved to a subdirectory such as packaging/.

*.appdata.xml files are used by AppStream powered software centers by Debian, Ubuntu, and other distros. It serves a similar role to inkscape.desktop, as I understand it.

The old autotools code referenced this file and appears to have been
processing it into inkscape.appdata.xml, but we don't appear to have any
cmake logic to process it.

0. The file can be moved to the packaging subdirectory without breaking anything currently.

1. cmake rules need added to generate ${CMAKE_BUILD_DIR}/inkscape.appdata.xml from ${CMAKE_SOURCE_DIR}/packaging/inkscape.appdata.xml.in, as autotools had been doing.

2. (Optional) The upstram-contact_at_email.com address should probably just be dropped. This is provided as a way for distributors to contact us about problems in the appdata file, but I suspect it'd be better to leave it undefined. They can reach us through inkscape-devel@ or the bug tracker or other normal channels.

3. (Optional) The file includes a link to a screenshot of an out of date version of inkscape. We should point to a more modern screenshot, perhaps hosted at inkscape.org. The AppStream specification has some guidelines of the screenshot size and layout.

4. (Optional) Since both inkscape.desktop and inkscape.appdata.xml (and probably other packaging files) provide essentially the same data but in different formats, since we're already processing the files with cmake it might be worthwhile to centralize all the links, descriptions, licenses, etc. in CMake and have it all be inserted into these files, so if we want to introduce an updated project description we can do it in one central place.