Display outline around dragged selection

[Kully]

Sometimes, it's not always obvious what pixels you are dragging when copying a selection.

This video demonstrates this issue, as get 'blocked' by being able to drag any further right because there are blue pixels in the way.

https://github.com/Kully/pixel-paint/assets/10369095/25ff1b53-fa31-4f3d-b3bb-2c46413e4221

To help this, it would be useful to show a line around the selection boundaries. (maybe a dotted yellow line?)