Enable the use of named colors

[TimJentzsch]

Objective

Close #1.

It's usually a bad idea to hard-code colors, as it makes it harder to use a consistent color-scheme throughout the app.
Instead, we should allow named colors to be used.

Solution

• A new ColorMap resource can be used to define color names
• The names can be used instead of hex values in the tags: [c=primary]
• Should the color for a name be updated, the update is efficiently applied to all usages of that name, without reconstructing the UI hierarchy