Add splitting (and resizing) to all types of clips (#7477)

Allow for splitting and resizing all types of clips (automation, MIDI, sample, pattern, etc) using the knife tool in the Song Editor.

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Co-authored-by: szeli1 <143485814+szeli1@users.noreply.github.com>
Co-authored-by: Dalton Messmer <messmer.dalton@gmail.com>

data/themes/classic/style.css

@@ -761,6 +761,7 @@ lmms--gui--ClipView   {
 	qproperty-textBackgroundColor: rgba(0, 0, 0, 75);
 	qproperty-textShadowColor: rgb( 0, 0, 0 );
 	qproperty-gradient: true; /* boolean property, set true to have a gradient */
+	qproperty-markerColor: rgb(0, 0, 0);
 	/* finger tip offset of cursor */
 	qproperty-mouseHotspotHand: 3px 3px;
 	qproperty-mouseHotspotKnife: 0px 0px;

data/themes/default/style.css

@@ -804,6 +804,7 @@ lmms--gui--ClipView   {
 	qproperty-textBackgroundColor: rgba(0, 0, 0, 75);
 	qproperty-textShadowColor: rgba(0,0,0,200);
 	qproperty-gradient: false; /* boolean property, set true to have a gradient */
+	qproperty-markerColor: rgb(0, 0, 0);
 	/* finger tip offset of cursor */
 	qproperty-mouseHotspotHand: 7px 2px;
 	qproperty-mouseHotspotKnife: 0px 0px;