Set the default font family for EPub's according to the user preference
Summary: It's impossible to define the font family for EPub using backend settings. This patch tries to take them into account.
Test Plan:
- Compile and install.
- Open an EPub file.
- Choose some custom font using the "Settings -> Configure Backends..." menu item.
- Close the Epub file with Ctrl + W and open it again.
- The new chosen font should be applied to the content.