imageView: Use custom slide widget and IndexSet

Use IndexSet to allow direct access by Path and also allow
an arbitrary ordering that was not possible with Vec. Also
add monitoring of file changes in the directory.

Use a custom LpSlidingView instead of AdwCarousel because
Carousel was never designed for this job and suffered of
bugs. LpSlidingView does not support swiping gestures yet
since they currently don't work yet anyways #24.

Use a new algorithm to fill the LpSlidingView that is
stateless and covers all potential changes in the directory.
This not only fixes bugs but would also work with changed
sorting orders.

This also adds build time template XML validation since it
was useful for the refactoring #46.

Closes #9, #11, #30, #33, #42

src/image_metadata/obj.rs

@@ -7,16 +7,17 @@ use gtk::glib;
 use once_cell::sync::Lazy;
 
 use std::cell::RefCell;
+use std::path::Path;
 
 glib::wrapper! {
     pub struct LpImageMetadata(ObjectSubclass<imp::LpImageMetadata>);
 }
 
 impl LpImageMetadata {
-    pub fn load(file: &gio::File) -> Self {
+    pub fn load(path: &Path) -> Self {
         let obj = glib::Object::new::<Self>(&[]);
 
-        let metadata = ImageMetadata::load(file);
+        let metadata = ImageMetadata::load(path);
         obj.imp().metadata.replace(metadata);
 
         obj