Intuitive opacity / alpha values in Cel properties and color pickers

[enol]

Adding alpha effects with the current Opacity bar when double-clicking over a Cel is not very efficient because you have to make an approximation to the desired level of opacity from 0 to 255.

I think that adding a box to change the percentage of the opacity with the keyboard can be very handful and people would be able to obtain a more precise effect on the range between 0% and 100% (more common values for drawing programs).

Aseprite 1.2 -beta9 (installer from Gumroad)

[dacap]

Requested here too:

• https://community.aseprite.org/t/100-instead-of-256/1916
• https://community.aseprite.org/t/feature-request-add-option-to-display-opacity-in-percent/4264

[dacap]

Something requested in ticket 3452 is the same 0%-100% range for RGB values.