A nice improvement just got merged into the master of TYPO3 CMS! Just one line has been added but it makes the live of an integrator a bit easier.
See the animated gif below for a short impression:
This is not a new feature but already possible since quite a while. If you need something like that as well, just use the following TCA configuration:
1 'enableMultiSelectFilterTextfield' => TRUE,
If you want to get to know more kind of hidden features of the TCA, please take a look at the styleguide which covers nearly all possible options: github.com/7elix/TYPO3.CMS.Styleguide