Retain cycling issues fixed

[Adiletkozha]

1. There was retain cycle in line inside PlayerSlider.swift:
   issue:var delegate: PlayerSliderProtocol?
   fix: weak delegate: PlayerSliderProtocol?
2. Problem also related to retain cycles inside block which is inside prefetchItems function within PandoraPlayer.swift
   issue: item = DispatchWorkItem(block: {
   fix: item = DispatchWorkItem(block: {[unowned self] in
3. targetColumn value inside CenterCellCollectionViewFlowLayout had been computing incorrectly
   issue: let targetColumn = floor(proposedContentOffset.x/w + 1)
   fix: let targetColumn = floor(proposedContentOffset.x/(w+self.minimumLineSpacing) + 1)