and it takes up alot of space on the toolbar.

So i copied the "1:1" zoom button and changed the text to "Fit",
the font Ubuntu Bold looked the most similar.

I tried matching the current buttons size (32x32) and position.
This has no transparancy,
so it will not look very good with a darker theme.
A toggle button suggestion:
Maybe there could be a zoom button that toggles between
100% and fit image to window,
it could change the icon depending on current zoom level.
If the image is zoomed at 100%,
the button has a "Fit" icon and it runs the command:
cmd_fitAll
otherwise if the zoom level is greater or smaller than 100%,
the button has a "1:1" icon and it runs the command:
cmd_zoom100