Only use fonts which are also installed on the system

Currently neptune3 uses "Dejavu Sans", while this font is known to be installed on most systems, it is not installed on ALL of them. This includes for example macos. Because the font is not installed the complete font database is checked, which takes very long and causes a very slow startup, which has some additional side effects.

The best approach is to provide the font we want to use with neptune and explictly load this, e.g. like the QtQuickControls examples are doing.