Memory leak select

[Rincelent]

Correct a small memory leak happening when destroying select widget

[flozz]

The changes looks good so far. There is only some coding style issue:

$ grunt jshint
Running "jshint:all" (jshint) task

   src/composite/select.js
    117 |        if (this.__displayValue) this.__displayValue.destroy();
                                          ^ Expected '{' and instead saw 'this'.
    329 |        if (this.__displayValue) this.__displayValue.destroy();
                                          ^ Expected '{' and instead saw 'this'.
    330 |        if (this.__popupMenu) this.__popupMenu.destroy();
                                       ^ Expected '{' and instead saw 'this'.

>> 3 errors in 54 files

[flozz]

Thank you :)