v3 Customizer allow edit rather than overwrite

Created by: jeme

Just an idea.

Make it so that when you are about to enter text into the text boxes, rather than putting the cursor at the start and deleting the old content on type, you would instead mark it all when you enter a text box, and the user could then "unmark" a portion to merely edit the value slightly.

This is properly mostly useful for the "fonts" section, where you might not wan't to define your own complete set of font's but merely add or change a single of them and put that in higher priority.

Right now you have to retype all the font's, including the ones already there as defaults...

It's quite a minor thing, but it should also be a minor thing to fix I believe.

Cheers.