** Performance improvement **

-> improved performances and fixed the bug where the user couldn't deselect all in filtered mode
-> added the class multiselect-item to all plugin-specific items
-> fixed an incorrect CSS selector

Signed-off-by: Tyf0x <Tyf0x@users.noreply.github.com>
** Updated the clearSelection method to use deselectall() **
** Further performance improvement **

-> Now only filters the options to select if not all them have to be selected. This means to select all should be faster when there is no filter used.
** Bugfix **
** Bugfix & selector performance improvement **

-> Fixed a bug where disabled options where incorrectly selected using the 'select-all'
-> Changed the jQuery .not() selector in favor of the css native :not() to slightly improve performance
** Bugfix **

-> The deselectall function was deselecting all options regardless of their visibility. It now takes it into account.
Merge pull request #279 from Tyf0x/perf

** Performance improvement ** - based on thorst #268 contribution.