Add Nord, Dracula, Solarized and Gruvbox themes

[varbhat]

@hecrj , This PR is accordingly to how we discussed in Discord.

This PR does two things:

1. makes color! macro return Color directly instead of invoking another function which does it. This way, we can use color! macro to create constant colors.
2. Adds the following palette and themes:
   • Dracula : https://draculatheme.com/
   • Nord: https://www.nordtheme.com/#palettes-modularity
   • Solarized (light and dark): https://ethanschoonover.com/solarized/
   • Gruvbox (light and dark): https://github.com/morhetz/gruvbox

[liigo]

Dracula, not Drakula.
Most of them are just code syntax highlightings, not really GUI themes. I don't think they are fit well for Iced.

[varbhat]

@liigo , Except for Gruvbox, other themes are color palettes which are already utilised by many non code-syntax apps.
And please run the demo. These look great and according to me, these are good additions.

And @hecrj wanted other color schemes than default light/dark, so i think it might be worthy to consider this.

[liigo]

@varbhat Color palettes can be used to create UI themes, but palettes themself are not UI themes. I suggest you create your own new themes, and give them new names, but don't use the name of those palettes. And maybe, they will reside in third-party crates. (After that for a long while, some of them that's very popular maybe have good chances to be merged into core crate as built-in themes, or replace existing light/dark themes.)

[varbhat]

@hecrj , rustfmt'd. try again

[varbhat]

@hecrj I am adding catppuccin (https://github.com/catppuccin/catppuccin) themes and palettes too.

[hecrj]

Thanks! 🥳

[hecrj]

@varbhat Open a new PR for that one!