AppMan: Implement support for a DESKTOP_FILE_HINT env var, with same purpose as desktop_file_hint

[Gerry Boland]

Fact: the --desktop_file_hint=/path/to/desktopFile.desktop command line parameter is a hack to enforce tight coupling of a process with its metadata (the desktop file contents). It should not be used in production, is only for convenience in testing.

But many applications (rightly) exit at startup when supplied with a command line parameter they do not support .

For Qt upstreaming, we want to be able to run the existing QTestLib-based tests against QtMirCompositor. But they all currently quit due to the --desktop_file_hint switch.

Adding support for an environment variable DESKTOP_FILE_HINT with the same purpose as the command line switch would fix this issue.

There are also places on Ubuntu which (ab)use --desktop_file_hint to avoid having to launch their tests with upstart. I want to flush these out, so eventually removing support for this switch would achieve that - so this env var would be handy fallback for us.