Add the possibility to change font size

[ea4k]

Small letters may be difficult to read in big/high definition screen.

[ikbenkous]

https://doc.qt.io/qt-5/highdpi.html

Traditionally, to support high DPI, Qt scales fonts automatically and provides a DPI value that application code can use to scale the rest of the UI.

Are changes required or does klog automatically scale the UI? I can't check here.