Consider Qt's fallback search paths when finding icons (#259)
Consider Qt's fallback search paths when finding icons (#259)

Closes https://github.com/lxqt/libqtxdg/issues/258

Notes:

 * For respecting Freedesktop standards, Qt's fallback search paths are taken into account only after inherited paths.
 * Colorizing of SVG icons is ignored with Qt's fallback paths because it depends on icon themes, not on paths.
 * To test it, you could do something like this in a code:

```c++
    QIcon::setFallbackSearchPaths(QIcon::fallbackSearchPaths() << PARENT_DIR_OF_EXTRA_ICON);
    anAction->setIcon(QIcon::fromTheme(EXTRA_ICON_NAME));
```
libqtxdg/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,lxde/libqtxdg