Slider: add support for crossing multiple handles

[hsyl20]

With this patch, a slider which has both "range = true" and "allowCrossingHandles = true" automatically determines the handlers starting and stopping the range. Handlers can freely cross each other.

Follow-up of #1869

[hsyl20]

I need some help with the test: I can't simulate a handle drag event (commented code in the test). it dies with:

>> slider: options - allowCrossingHandles
>> Message: Died on test #2 file:///home/hsyl20/repo/jquery-ui/tests/unit/slider/options.js:404:11
>> execCb@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:1658:38
>> check@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:874:57
>> enable@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:1151:27
>> init@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:782:32
>> callGetModule@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:1178:67
>> completeLoad@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:1552:34
>> onScriptLoad@file:///home/hsyl20/repo/jquery-ui/external/requirejs/require.js:1679:41: Attempting to change access mechanism for an unconfigurable property.
>> Actual: null
>> Expected: undefined
>> defineProperty@[native code]
>> mouseEvent@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:122:26
>> createEvent@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:83:26
>> simulateEvent@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:73:31
>> simulateDrag@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:307:21
>> simulate@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:30:17
>> file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:19:17
>> each@file:///home/hsyl20/repo/jquery-ui/external/jquery-1.12.4/jquery.js:370:23
>> each@file:///home/hsyl20/repo/jquery-ui/external/jquery-1.12.4/jquery.js:137:21
>> simulate@file:///home/hsyl20/repo/jquery-ui/external/jquery-simulate/jquery.simulate.js:18:18
>> file:///home/hsyl20/repo/jquery-ui/tests/unit/slider/options.js:418:26