Application icon cleanup

Ensure images used for application icon are a "best quality" image
with consistent square size. A sharp, square image is vital for downstream
usage/installation.

File `images/logo.svg` was taken as canonical source. File was edited in
inkscape. It's document properties were cleaned up(incl. image metadata).
Document viewport was made to be a large size and square(256px^2).
Image itself was resized to ensure an 8px margin.

File `images/logo.png` was created from svg (inkscape `export to png`
command). Image was generated with high compression as well as ensuring
an alpha layer and square sizing.

File `src/icons/sqlitebrowser.png` was copied from `images/logo.png`.

Author: scottfurry