Feature Request: Ability to toggle/set Lock Layer Size Proportions
Right now it's a pain to:
- check whether a symbol has 'constrain layer size proportions' enabled
- use mouse/trackpad to click the little icon to toggle it

Would be great to have a feature to:
- toggle 'Toggle Layer Size Proportion Lock'
Or:
- 'Lock Layer Size Proportion'
- 'Unlock Layer Size Proportion'

(Not sure about the best terminology. Not sure what terminology Sketch uses itself).

Basically, a feature that would click this icon:
<img width="447" alt="lock layer proportions" src="https://user-images.githubusercontent.com/2269563/42454357-9617b5a4-838f-11e8-80ee-8e51709c6b65.png">
