Avoid using the fall-back circle icon

[dangvd]

There are 2 main cases that Crystal Dock has to fall back to the circle icon:

1. If the app is found on the Application List, but QIcon::fromTheme returns a null icon
2. If the app is not found on the Application List

To fix:

1. For this case, we need to load the icon displayed in the Application List, even if it's not present in the current theme
2. For this case, we need to retrieve the task icon. For KDE Plasma 6, we should use get_icon