Custom font support

[hecrj]

Depends on #52.

This PR implements a basic Font type and a Text::font method. End-users can now build GUIs using their own custom fonts. This is useful if you want to render icons, for instance!

It also refactors a bit the text pipeline in iced_wgpu.

Changelog

Added

• The Font type in iced_core with a Default and External variant.
• A Text::font method. It sets the Font to use to render the Text.