Add qwt for focal to qwt5 osdeps

[0Nel]

Indeed qwt5 for qt4 is nonexistent on ubuntu 20.04, which requires qt5.

However packages that require qwt on ubuntu 20.04 build fine with libqwt-qt5-dev

At first qwt5 did not seem to be the correct key, however since qwt is defined for fedora, opensuse and darwin as well I didn't open up a new entry.

[pierrewillenbrockdfki]

qwt5 is not about qt5 support, it is about qwt5 support; qwt5 generally is compiled for qt4, while qwt6 generally is compiled for qt5.

libqwt-qt5-dev evidently is qwt6, it installs Qt5Qwt6.pc.

[planthaber]

So there should be a new entry

qwt6:
    debian,ubuntu:
        default: libqwt-qt5-dev

or we create an additional qwt: dependency in case qwt5 and 6 are compatible. For pacages that qupport qt4 and 5

qwt5:
    debian,ubuntu:
        default: libqwt5-qt4-dev
        '20.04': nonexistent
    fedora,opensuse: qwt-devel
    darwin: qwt
    arch: qwt5

qwt:
    debian,ubuntu:
        default: libqwt5-qt4-dev
        '20.04': libqwt-qt5-dev
    fedora,opensuse: qwt-devel
    darwin: qwt
    arch: qwt5