Fix using font

[Ptosiek]

Hi!
Not sure if the feature is used?

1. The font "Yantramanav-Black.ttf" is not source so I remove the code handling it.
2. Using self.app.setFont(font), instead of setting it manually to some components
3. Can't test on MacOS, so the full_path of the font is given to QtGui.QFontDatabase.addApplicationFont
   via os.path.join(os.getcwd(), "fonts", self.config.G_FONT_FILE). But don't know if it's necessary

[hishizuka]

Hi @Ptosiek ,
Thank you for your PR.

1 I used to specify a default font. Now you can specify it in setting.conf, so remove it.
2. OK, thanks.
3. MacOS font specification was difficult and its test code was left behind. Let's delete it.