feat: Add unique iconography for each note type

[dacoaster]

Feature Request

I think there's power in using uniquely coloured icons to differentiate between note types. For example, as you scan a report you can get a sense of the sort of things that have been captured and then dive into the detail.

Perhaps a red exclamation icon for "Problem". Or yellow lightbulb icon for "Idea".

I think this will also make it easier for someone to quickly add a note type. They'll see the coloured icon and easily grab it instead of just relying on the text label.

[dacoaster]

Fixed in #48

[dacoaster]

Added in v0.6.0