Allow `width` property in styles

This one property will make responsive websites in Bubble SO MUCH SIMPLER.

Add the ability to set width via condition on a style. Doing so will mean that I can make all elements that match a particular style (e.g. menu item) switch to 100% width when the page width is small, which means I can set things up to be responsive.

Right now this is possible but I have to individually set up the condition on every single element, which is very time consuming. And if I need to make a change after I’ve put in all that work, God help me.