fix(MdTable): emit selected / update event only when selected item(s) really changed

[VdustR]

fix #1559

[VdustR]

Additionally, I want to make another PR to implement model comparing in utils. Maybe named MdModelComparator?

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, I want to implement a method compare(a, b) followed these rules:

• undefined = null
• null != 0 != NaN != ''
• 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 could be write in documentation to let developers know how it work.

@marcosmoura , @Samuell1
How do you think?

[marcosmoura]

I think this method is way complex. Maybe we can break it in some small functions to better describe it.

[marcosmoura]

Like: val.length !== old.length) ? true : !val.every((item, index) => item == old[index]

[marcosmoura]

@VdustR I think it would be good. I want to make less code repetition.