Dynamic type support + Appearance improvements

[dive]

Hello there,

Changes

This is the base implementation of the new cell to render tasks with the following features:

• Full dynamic type support;
• Dark and light themes support;
• Easy to customise (build on top on NSAttributedString);
• Show additional information: scheduled, deadlines, notes;
• Done tasks are greyed out.

There is a huge room for improvements, but I am going to apply them later to keep changesets meaningful.

Note for a reviewer

The only concern I have is about beforeText/afterText. I have removed support for both because I did not find any references and how they used. Maybe I am wrong and we need these fields.

These changes resolve #204.

Examples

Small video is available here.

Small Font	XXL Font	Light Theme

iPad Pro 11 Dark	iPad Pro 11 Dark

[webframp]

This looks great, merging.

Sidenote: Awesome inclusion of the video, very helpful. I don't know if you watched it 100+ times but imgur says that many views have happened. So maybe there's a few interested lurkers on this repo 😀