new util: MdModelComparator

[VdustR]

All components have selection could call the function to check the value or array is changed or not, including MdSelect, MdTable, MdRadio.

for single comparing, there will be a method compare(a, b) followed these rules:

undefined = null
null != 0 != NaN != '' != false
0 = '0', 1 = '1', ...
any non-object-value != any object, an object != another object
for multiple comparing, I want to implement a method compareArray(a, b) followed these rules:

[] = null = undefined
regarding sorting,['a', 'b'] = ['b', 'a'].
including objects, [a, b] = [b, a]. I'll using find index to implement that.
values in array follow single comparing rule, like: [null] = [undefined]
These rules will be wrote in documentation to let developers know how it work.

Issue from #1585 (comment)