Dynamic Font Support

[aubaugh]

Hello folks - I'm looking to add support to the embedded-text crate to allow for handling multiple types of fonts dynamically within a TextBox. #152

Some font types I would ultimately like to support include:

• MonoFont from embedded-graphics
• U8g2Font from u8g2-fonts
• BdfFont from embedded-graphics/bdf
• BitmapFont from mplusfonts

I was thinking of adding a new trait to embedded-graphics named FontSetter which would have a corresponding set_font method similar to the existing CharacterStyle trait.

I've come to a conclusion that in order to support downcasting of various font types that we would need to implement one of the three solutions:

• Heap allocation to represent general fonts
• Leveraging the type id and using an unsafe block to downcast the font
• Implementing an enum of various FontTypes that the embedded-graphics crate would need to depend on

I was thinking the third implementation would be desirable as it wouldn't require heap allocation or unsafe code, but it would result in limited extensibility as future font types would need to update embedded-graphics to update the FontType enum.

Any feedback would be much appreciated