Added inline searching for multiple selects

This adds a search box to the main container for multiple select
boxes, similar to the what the old version had. This can still be
swapped out such that the search box displays above the results
like it previously did.

The placeholder for multiple selects will also now use the input
attribute of the search box instead of generating a custom element.

This also changes the selector for the search container to
`.select2-search`, which is consistent with the old class and
reduces the complexity of the selectors.

This fixes an issue with how decorators worked, where the constructor
from the parent class would clobber the custom constructor that is
generated for the new decorated class. This has been fixed by
excluding the constructor from the list of fields which are
transferred when decorating classes.
Automatically resize the search box

Now the search box is automatically resized for placeholders to
fit the available space, so we don't need to worry about the
placeholder not being visible. When the placeholder doesn't need
to be visible, the size of the search box is determined based on
the length of the current search term.