style: unify identifier naming per updated conventions

Automated with clang-tidy readability-identifier-naming (config added to
.clang-tidy) plus scripted passes, per the updated rules now documented
in CONTRIBUTING.md:

- types (class/struct/enum/alias/template params): PascalCase
- functions, variables, members: snake_case (incl. rational -> Rational)
- private/protected members: trailing underscore; static member
  variables likewise (instance_, available_themes_)
- constants and enum values: snake_case (kLinear -> k_linear,
  F32P -> f32p); ALL_CAPS reserved for macros
- macros: OAK_ prefix (OLIVE_ADD_TEST/OLIVE_ASSERT/OLIVE_CONFIG ->
  OAK_ADD_TEST/OAK_ASSERT/OAK_CONFIG, GL_PREAMBLE -> OAK_GL_PREAMBLE,
  include guards -> OAK_*)
- file names: all lowercase (Current/Plugin/OliveHost/OliveClip/
  OlivePluginInstance -> current/plugin/olivehost/oliveclip/
  oliveplugininstance)
- getters share the member name sans underscore, setters set_foo()
- Qt and third-party (OpenFX) virtual overrides and framework callbacks
  keep their original names (exempt in .clang-tidy)

Manual follow-ups required where automation could not reach:
- string-based QMetaObject/SIGNAL/SLOT references updated to renamed
  methods (AddTask, CreatedFile, DeleteSpecificFile, moveSelectionUp, ...)
- macro bodies referencing renamed methods (OLIVE_CONFIG,
  NODE_DEFAULT_DESTRUCTOR, MANAGEDDISPLAYWIDGET_*)
- self-shadowing locals renamed where signals/methods became same-named
  (size_changed, worker_count, selected_items, import param, filters)
- third_party OFX member/namespace usages restored (OFX::Host::*,
  _created, _clipPrefsDirty, createInstance, clearPersistentMessage)
- STL protocol aliases restored (const_iterator) with .clang-tidy
  ignore rules; qHash overloads restored

Full build and test suite pass: ctest 4/4, ~1960 gtest cases green.

app/undo/undostack.h

@@ -19,8 +19,8 @@
 
 ***/
 
-#ifndef UNDOSTACK_H
-#define UNDOSTACK_H
+#ifndef OAK_UNDOSTACK_H
+#define OAK_UNDOSTACK_H
 
 #include <QAction>
 #include <QAbstractItemModel>
@@ -44,14 +44,14 @@ public:
 
 	void clear();
 
-	bool CanUndo() const;
+	bool can_undo() const;
 
-	bool CanRedo() const
+	bool can_redo() const
 	{
 		return !undone_commands_.empty();
 	}
 
-	void UpdateActions();
+	void update_actions();
 
 	QAction *GetUndoAction()
 	{
@@ -79,7 +79,7 @@ public:
 	hasChildren(const QModelIndex &parent = QModelIndex()) const override;
 
 signals:
-	void indexChanged(int i);
+	void index_changed(int i);
 
 public slots:
 	void undo();
@@ -87,7 +87,7 @@ public slots:
 	void redo();
 
 private:
-	static const int kMaxUndoCommands;
+	static const int k_max_undo_commands;
 
 	struct CommandEntry {
 		UndoCommand *command;
@@ -105,4 +105,4 @@ private:
 
 }
 
-#endif // UNDOSTACK_H
+#endif // OAK_UNDOSTACK_H