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/Cargo.toml

@@ -20,6 +20,7 @@ little_exif = "0.4"
 imageproc = "0.25"
 ab_glyph = "0.2"
 dirs = "6"
+notify = "7"
 
 [dev-dependencies]
 tempfile = "3"