Set a better default size (height and width) for the event editor dialog

Fixes issue #602 (closed). The values are still hardcoded instead of measuring the available space (so it's not the most elegant, "smartest" solution in my view, but oh well), but it's better than nothing in the meantime.

Decent WMs (like Mutter) size the window correctly if width/height exceed the screen, because the default size are not true constraints, merely suggestions for WMs, AFAIU.

Note that I have only tested this on GNOME 43, with a 2560x1080 screen and a 1024x768 screen.

With this, the Notes field also expands vertically to fill the space.