VS: Generate default AppxManifest in a per-target directory

Avoid clashes when more than one executable are being generated in the
same folder.
VS: Place missing artifacts in per-target intermediate directory

Also revise missing file generation code to simplify layout and
encode characters for XML correctly.
Merge topic 'vs-windows-apps' into next

f1e9fa3c VS: Place missing artifacts in per-target intermediate directory
22a89b7a VS: Generate default AppxManifest in a per-target directory