Show % options in editor size controls

[karaggeorge]

I love this in Gifski:

Even if we have less options, I think having a select with % of the original size is super useful. Most of the changes I make in the editor controls are /2 or /10 I think that'll also help with the whole retina size issue.

We can add some steps like:

• Original
• 75%
• 50%
• 33%
• 25%
• 20%
• 10%

UI-wise, it could be a small select next to the size inputs that just shows the % value (rounded) and when clicked the menu can have the full length % options with the actual size, same as in the screenshot above