libqwt: new package

Patch initially written by Thomas Petazzoni, then reworked and updated
by Laurent Gonzalez, and finally cleaned up by Thomas again.

[Peter: generic-package, deps, download, rename to qwt, move under Qt]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>