icons: Add more sizes for xfce4-timer-plugin.png

I specifically want the 16x16 icon, this fixes weird icon size for
"Add new alarm" menu item under a Xfce setup with 2x window scaling.

The icons are generated with e.g.:
rsvg-convert -w 16 xfce4-timer-plugin.svg -o xfce4-timer-plugin-16x16.png