fix number parsing in calculator (support e notation)

this fixes contrast/brightness going bonkers
You need to include glib/gprintf.h:
```
The recommended way of using GLib has always been to only include the toplevel headers glib.h, glib-object.h, gio.h. Starting with 2.32, GLib enforces this by generating an error when individual headers are directly included.
```
https://developer.gnome.org/glib/stable/glib-compiling.html