Add RemoteDesktop to the image.

There are some rough edges, but this can be useful and deserves more
exposure.
SetTime_t: Use the sign-fixed modulo we computed

* In C++03 and earlier, the sign of the modulo result is implementation
defined (this was fixed in C++11).
* Setting a negative time is not what we want here, so make sure we use
the proper value.
Add a first test for BDateTime.

* We need to write more of those.
don't notify when a finished download is removed.

* Actually stop the download when the target file is deleted
* Check the download state before sending a notification or changing the
progress bar color.

Fixes #10907.