Base+ImageViewer: Restore correct app icon

Restore ImageViewer's new application icon, which was accidentally
changed back to using the filetype icon by e800605.
Additionally, use the correct icon in the ImageViewer manpage.

Base/usr/share/man/man1/Applications/ImageViewer.md

@@ -1,6 +1,6 @@
 ## Name
 
-![Icon](/res/icons/16x16/filetype-image.png) Image Viewer - SerenityOS image viewer
+![Icon](/res/icons/16x16/app-image-viewer.png) Image Viewer - SerenityOS image viewer
 
 [Open](file:///bin/ImageViewer)
 
@@ -29,4 +29,3 @@ ImageViewer is even smart enough to detect other images and display them when cl
 ```sh
 $ ImageViewer /res/graphics/buggie.png
 ```
-

Userland/Applications/ImageViewer/CMakeLists.txt

@@ -11,5 +11,5 @@ set(SOURCES
         ViewWidget.cpp
 )
 
-serenity_app(ImageViewer ICON filetype-image)
+serenity_app(ImageViewer ICON app-image-viewer)
 target_link_libraries(ImageViewer PRIVATE LibCore LibDesktop LibFileSystemAccessClient LibGUI LibGfx LibConfig LibImageDecoderClient LibMain LibURL)