Fix SVG icons not being displayed on Windows.

Fix SVG icons not being displayed on Windows.

On Windows (where we use a static Qt) we need to include plugins
(in this case "qsvg" a.k.a. libqsvg.a from plugins/imageformats/),
otherwise SVGs (such as various icons in PulseView) cannot be displayed.
For this we need to use Q_IMPORT_PLUGIN(qsvg) in the code, pass the
QT_STATICPLUGIN flag to the compiler, and link against "-lqsvg".

This, in turn, requires that we also link against the similarly named QtSvg
component/lib (${QT_QTSVG_LIBRARY} a.k.a libQtSvg.a).

See also: https://qt-project.org/doc/qt-4.8/plugins-howto.html#static-plugins

This fixes bug #239.