Define getDefaultSettings() in JSlider
Update JSlider to get/set customTotalWidth
Rename #total-width to #custom-total-width
Update getCss() to use customTotalWidth
Update jsliderSpec to check customTotalWidth
Describe setters
Reimplement setters to return true/false
Improve setId() to forbid numeric strings
Define initAdvancedInputs() in ui.js
Define isAdvancedEdit() in ui.js
Clean UI's canUpdate() using setSettings()
Clean UI's getOptions(), assign customTotalWidth

* rename getOptions() to getSettings()
* use isAdvancedEdit()
Prefix settings in setSettings()