feat: add CompositeIcon struct for nesting SVGs

Cargo.lock

@@ -23,6 +23,12 @@ version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
 
+[[package]]
+name = "base64"
+version = "0.23.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b25655df2c3cdd83c5e5b293b88acd880332b2ddadd7c30ac43144fdc0033da9"
+
 [[package]]
 name = "bitflags"
 version = "2.13.1"
@@ -402,6 +408,7 @@ name = "icon-color-tool"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "base64",
  "gtk4",
  "regex",
  "serde",