image: Add zoom and scrolling support

Zoom via
- Scroll event controller
- Zoom gesture
- Overlay buttons
- Fixed zoom level shortcuts (1=100%, 2=200%)
Scrolling via
- Scrolled window
- Drag gesture
Changes
- Implement best-fit via calculating a zoom level
- Gesture for carousel no longer working because of scrolled window
- Split control overlays into two to not steal inputs in area between

Closes #17

Cargo.toml

@@ -23,7 +23,7 @@ features = ["gtk4"]
 [dependencies.gtk]
 package = "gtk4"
 version = "0.5.1"
-features = ["v4_6"]
+features = ["v4_8"]
 
 [dependencies.gio]
 package = "gio"