Enable variables in various tools precise input fields
Description of the feature
I assume that there may be two types of inputs to mathematic expressions - numbers from user's imagination - like "I want to divide this image in 3 parts" or inputs that represent measurements of the image - like "I want to put this guide at the same distance from point X as layer Y upper left corner is from layer Z upper left corner".
Obviously, this is a use case for measure tool. What I am proposing is supposed to 1) make typical operations faster 2) simplifying the workflow in case there are several measurements to be taken.
Few examples of the variables that may be useful - layer positions, layer sizes, distances between guides or guide intersections, font sizes in various font layers, list of measures taken with measure tool in going backward in time.
A complicated issue to be solved here would be naming - all this would be near impossible to use unless a good "discoverable" naming scheme would be invented.
Use cases
I would be happy to collect some input from the users (e.g. at /r/GIMP) to help determine whether what "variables"(needs better naming by the way) would they find useful.
@mitch @Jehan @prokoudine - what do you guys think of this?