feat(themes): generate base16 palette from wallpaper

This commit adds a new Wallpaper configuration option to the
WorkspaceConfig, allowing the user to specify a path to a wallpaper
image file, and to specify whether to generate a base16 palette from the
colours of that image file.

Theme generation is enabled by default when a wallpaper is selected.

A set of theme options can be given to customize the colours of various
borders and accents.

The themes generated are also plumbed through to the komorebi-bar.

The palette generation algorithm from "flavours" (which has been forked
and updated) is quite slow, so the outputs are cached to file in
DATA_DIR, and keyed by ThemeVariant (Light or Dark).

The Win32 COM API to set the desktop wallpaper is also quite slow,
however this calls is async so it doesn't block komorebi's main thread.

komorebi-themes/src/colour.rs

@@ -57,7 +57,7 @@ impl From<Colour> for Color32 {
 }
 
 #[derive(Debug, Copy, Clone, Serialize, Deserialize, PartialEq)]
-pub struct Hex(HexColor);
+pub struct Hex(pub HexColor);
 
 #[cfg(feature = "schemars")]
 impl schemars::JsonSchema for Hex {