Add desktop and icon files and install into the correct place

Remove 48x48 icon; use 256x256 icon only.
Remove icons from the executable.
No need to call gtk_window_set_default_icon_name() anymore.
Add desktop file.
Add install and uninstall Makefile targets.
Update USAGE.md with installation instructions.
Thanks to @szszoke for figuring this one out!

Fixes #2.

src/main.c

@@ -30,7 +30,6 @@ static void load_css(void) {
 // gtk init
 
 static void startup(GtkApplication *app, gpointer user_data) {
-  gtk_window_set_default_icon_name("alsa-scarlett-gui");
   gtk_application_set_menubar(app, G_MENU_MODEL(create_app_menu(app)));
 
   alsa_inotify_init();