Add dot SVG icons

Add dot SVG icons

The advantage of SVG dots over Unicode dots is that their size is
fixed.  With Unicode dots, the size varies with the font.

Dot icons can be useful for status indicator: green dot for online,
red dot for offline, etc.

* etc/images/symbols/dot_large_16.pbm:
* etc/images/symbols/dot_large_16.svg:
* etc/images/symbols/dot_medium_16.pbm:
* etc/images/symbols/dot_medium_16.svg:
* etc/images/symbols/dot_small_16.pbm:
* etc/images/symbols/dot_small_16.svg: New file.