rework ClientModel::getBlockSource() + BitcoinGUI::setNumBlocks()

- updates ClientModel::getBlockSource() to return all available states and
  sorts enum BlockSource in order of usage cases (none default, then reindex,
  import and network)
- updates BitcoinGUI::setNumBlocks() to better use getBlockSource() and
  also adds a message, when we have NO block source available

src/qt/clientmodel.h

@@ -15,9 +15,9 @@ QT_END_NAMESPACE
 
 enum BlockSource {
     BLOCK_SOURCE_NONE,
-    BLOCK_SOURCE_NETWORK,
+    BLOCK_SOURCE_REINDEX,
     BLOCK_SOURCE_DISK,
-    BLOCK_SOURCE_REINDEX
+    BLOCK_SOURCE_NETWORK
 };
 
 /** Model for Bitcoin network client. */