Skins: fix remaining 'font leaks'

[ronso0]

now all widget fonts should be determined by skin stylesheets, i.e. impossible to override by OS settings.

For example when you choose a huge, italic serif type in OS font settings or with Qt5 Settings there should be no spot in Mixxx that is bigger than expected, italic or not the skin font.

Exceptions:

• table header font size, though all characters should be visible (not cut off) and the header height should be about 10% taller than the font
• context menu font size
• main menu bar font, size and style

[daschuer]

I cannot spot a regression. Is there anything special to test or can we just merge?

[ronso0]

tiny risk of regression (now even harder to notice if the OS uses a decent sans-serif).
I took a brief look at each skins, confirmed the spots where I noticed the issues are fixed, just didn't want to merge myself.

[daschuer]

Ok, thank you. Than I will hit the button.