Support for user-installed system fonts in addition to ReadiumCSS predefined typefaces

[danielweck]

Fixes #766
Fixes #765

Note that unlike r2-testapp-js which uses a code library (see below) to discover font faces installed on the operating system, Thorium's implementation allows the user to type in any arbitrary name (case insensitive, and sanitized to avoid breaking ReadiumCSS).

1. Lib used in the test app (pure Javascript): https://github.com/rBurgett/system-font-families
2. Alternative lib (has native code): https://github.com/foliojs/font-manager
3. Another lib (has native code): https://github.com/oldj/node-font-list

[danielweck]

Default publisher font:

[danielweck]

Dyslexia-friendly font (ReadiumCSS predefined typeface):

[danielweck]

Custom selection (... option):

[danielweck]

Atkinson Hyperlegible:

( https://brailleinstitute.org/freefont )

[danielweck]

WebDings and WingDings

WHY?!

[danielweck]

Comic Sans

(some people like it)

[danielweck]

Misc.

(case insensitive, some names contains spaces, and generally-speaking typeface names are automatically sanitized and normalized to trim leading and trailing whitespace characters, and to collapse inner whitespace too)