Fix prefix/postfix buttons disappearing when colReorder is also used

[demetris-manikas]

Hi.

I use both the colVis and colReorder plugins and I noticed that if a column has been moved then
all prefix/postfix buttons that I have defined along with the colvis button vanish.

Here is the definition I use

var button = {
        extend: 'colvis',
        text: 'Reset',
        columns: ':not(.colvis-ignore)',
        postfixButtons: [
            {
               extend: 'colvisRestore',
            }
        ]
    }

This PR addresses this issue.
Please let me know whether I should setup a showcase for this.
Thanks a lot for your time.

[AllanJard]

Many thanks for this! I'll review it properly before the next Buttons release. Are you happy for this to be included under the MIT license?

[demetris-manikas]

Yes of course I am.

[demetris-manikas]

I have also created an almost identical to colVis extension (renames mostly) for manipulating the searchable property.
Would you be interested in that as well?