luci-base: cbi.js: rework dropdown implementation

 - Refactor event handler closures into class methods and bind them instead
 - Fix quirk in dropdown placement calculation
 - Different dropdown placement strategy on touch devices
 - Broadcast custom "cbi-dropdown-change" event when value is changed
 - Implement setValues() method to alter dropdown selection
 - Prevent creating empty custom values

Signed-off-by: Jo-Philipp Wich <jo@mein.io>