[studio] Move UndoCommand implementation to its own file

src/olympic/studio/modlib/src/CMakeLists.txt

@@ -7,6 +7,7 @@ add_library(
 		popup.cpp
 		project.cpp
 		task.cpp
+		undocommand.cpp
 		undostack.cpp
 		filedialog_nfd.cpp
 )

src/olympic/studio/modlib/src/undocommand.cpp

@@ -0,0 +1,10 @@
+
+#include <studio/undocommand.hpp>
+
+namespace studio {
+
+bool UndoCommand::mergeWith(UndoCommand const*) noexcept {
+	return false;
+}
+
+}

src/olympic/studio/modlib/src/undostack.cpp

@@ -6,10 +6,6 @@
 
 namespace studio {
 
-bool UndoCommand::mergeWith(UndoCommand const*) noexcept {
-	return false;
-}
-
 void UndoStack::push(ox::UPtr<UndoCommand> &&cmd) noexcept {
 	for (auto const i = m_stackIdx; i < m_stack.size();) {
 		std::ignore = m_stack.erase(i);