Makefile: revert back to using wildcards for sources
TNode: new ABC for TreeNode-like classes

Currently a mix of CRTP and virtual functions
Source: Derive from & implement TNode "interface"
Block::findSources use Source's TNode::collectIf()
TestSource: use new collectIf() and childCount()
Source: remove old findSources() and findSourcesRec()

This has now been replaced by TNode::collectIf