MEDIUM: applet: Emit a warning when a legacy applet is spawned

MEDIUM: applet: Emit a warning when a legacy applet is spawned

To motivate developers to support the new applets API, a warning is now
emitted when a legacy applet is spawned. To not flood users, this warning is
only emitted once per legacy applet. To do so, the applet flag
APPLET_FL_WARNED was added. It is set when the warning is emitted.

Note that test and set on this flag are not performed via atomic operations.
So it is possible to have more than one warning for a given applet if it is
spawned in same time on several threads. At worrst, there is one warning per
thread.