CoverCache - emitting "requestRepaint()" when the caller is CoverDelegate + connect this signal to WTrackTableView::update()

It fix some issues with the traktor_view:
@kain88-de
- The covers don't get painted once they are loaded, you need to scroll again to issue another paint event
- If you click a row in the libraryview the cover is first overlayed with blue and after a second it shows the normal cover again while the rest of the row is still blue
implement https://bugs.launchpad.net/mixxx/+bug/1180052
CoverArtCache::changeCoverArt - rescale cover right after its loading
CoverArtCache::requestPixmap - removing unnecessary "requestRepaint" emission

The signal "requestPaint" will be used by widgets which are already calling "requestPixmap" in a paint event.
In this way, emit this signal into "requestPixmap" does not make sense.
Cosmetic fix - rename the parameter "bool fromDelegate" to "bool issueRepaint"
CoverCache - fixing bug introduced by the last changes in the "requestPixmap" parameters
Cosmetic fixes - renaming const vars using lowerCamelCase and starting them with the letter 'k'
Simplifying CoverArtDAO::deleteUnusedCoverArts() - doing everything at once
CoverArtDAO - cosmetic fix - following code convention
Simplifying db query of CoverArtDAO::deleteUnusedCoverArts()
Added EngineFilterLinkwtzRiley
Added filter analysis functions
Added a LinkwitzRiley8EQeffect stub
git added Linkwitz-Riley mix to the Eq effect
disabled IIR_ANALYSIS
Added gain ramping
Merge branch 'master' into linkwitzriley
Merge branch 'master' of https://github.com/mixxxdj/mixxx into coverArtSupport

Conflicts:
	src/soundsource.cpp
Cosmetic fix - database schema - removing tabs (using only spaces)
Cosmetic fix - removing tabs (using only spaces)