implemented new block system

Fixes recurring design issue that the Blocks were a frequent exception to the
DAG concept. The Blocks connecting to each others inputs/outputs while not
necessarily being "dependent" on each other to produce an image continually
causes issues while trying to create a rendering code path. This redesign
should provide a more "directed" approach to the directed acyclic graph.

app/widget/timelinewidget/tool/pointer.cpp

@@ -513,7 +513,7 @@ rational TimelineWidget::PointerTool::ValidateOutTrimming(rational movement,
     if (prevent_overwriting) {
       // Determine if there's a block in the way
       Block* next = block->next();
-      while (next != nullptr && next->type() != Block::kEnd) {
+      while (next != nullptr) {
         if (next->type() == Block::kClip) {
           latest_out = qMin(latest_out, next->in());
           break;