Add file watcher for watch folder functionality

inotify-based folder watcher using the notify crate that detects new
image files, ignores non-image files, and supports start/stop lifecycle.
WatchFolder config struct for preset-linked watched directories.

pixstrip-core/src/lib.rs

@@ -9,6 +9,7 @@ pub mod pipeline;
 pub mod preset;
 pub mod storage;
 pub mod types;
+pub mod watcher;
 
 pub fn version() -> &'static str {
     env!("CARGO_PKG_VERSION")