This fixes a bug with the search box where, when it had a placeholder,
it would expand the width of the selection container because it was
too large. This bug was specifically caused by the search box not
factoring in the padding surrounding it when caclualting the width
it needed to be, which resulted in the search box extending
outside of the selection container. This bug was easy to notice if
your Select2 was set to have 100% width and if the container it was
held within was not a block element.
This fixes the bug by switching to using `width()` for calculating
the search width instead of using `innerWidth()`, which ignored the
surrounding padding.
Fixes#5517Closes#5518
