Editable list of new image preset sizes

[arseniiv]

I wanted to add a 1920×1080 size to use for screencasting but found out that the custom size doesn’t seem to be retained between sessions ...ah wait, it does if there’s nothing in clipboard so Clipboard option doesn’t activate and doesn’t rewrite the size. Well anyway a list of custom sizes, even if editable just by hand in settings.xml would be nice to have (to add more than one custom format for later).

This is also connected to #994 — the last custom size used can then be taken as dimensions for the default blank image opened at startup, as saved custom sizes would be stable, being independent of it.

Also thanks for having palette loading/saving and supporting the gmp format! 🎨 Very handy.

[cameronwhite]

Adding a button in the New Image dialog to save the size as a preset would be reasonable, I think. And maybe also a button to save as the default for new images