Add placeholder text to numeric selectors

[wayfaringrob]

Currently only category selectors support placeholder text. I would appreciate the ability to include placeholder text when a given numeric filter is not active. At a glance, it may look like nearly all of these filters are turned on, and for the user, there's really no way to know for sure unless I click each one and check the sliders. For ease of use and peace of mind, I would like to be able to put "Filter inactive" or something similar in there in place of the values...

...like I can for category selectors ("Showing all routes" is my placeholder text):