Reduce number of recursively included headers

This should improve compilation speed.

Things changed:
* Prefer forward-declarations in headers.
* Move header-includes out of headers if possible.
* Move some functions definitions out of headers.
* Put some member variables into unique_ptrs (see Client).

src/unittest/mock_inventorymanager.h

@@ -18,6 +18,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 */
 
 #include <gamedef.h>
+#include <inventory.h>
 #include <inventorymanager.h>
 
 class MockInventoryManager : public InventoryManager

src/unittest/test_voxelalgorithms.cpp

@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "voxelalgorithms.h"
 #include "util/numeric.h"
 #include "dummymap.h"
+#include "nodedef.h"
 
 class TestVoxelAlgorithms : public TestBase {
 public: