Improve fonts settings

[ffoodd]

To explore (maybe):

• [ ] font picker should allow to preview font in the list (ala Word or Outlook).
• [ ] include .woff2 fonts instead of using data:, in extension — server still requires data:, probably? This is needed because the data: protocol is forbidden on some sites via Content-Security-Policy (e.g. github.com or MDN).
• [ ] give a shot to the fontLoading API to check whether a font is loaded or not, before trying top use it.
• [ ] Using some modern CSS, we could ensure every fonts available in Confort+ is applied with the exact same physical properties, thus reducing the visual reflow when switching from one font to another. Alex Barashkov wrote "Advanced web font optimization techniques" on pixelPoint blog and it feels really accurate.
• [ ] Chrome has a fontSettings API that may help reduce the amount of code needed in chromium-based browser extension.
• [ ] Firefox allows to drop custom fonts entirely through browserSettings.useDocumentFonts(), which could be an interesting behaviour for users who like their system font?