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/widget/history/historywidget.cpp

@@ -33,20 +33,20 @@ HistoryWidget::HistoryWidget(QWidget *parent)
 
 	this->setModel(stack_);
 	this->setRootIsDecorated(false);
-	connect(stack_, &UndoStack::indexChanged, this,
-			&HistoryWidget::indexChanged);
+	connect(stack_, &UndoStack::index_changed, this,
+			&HistoryWidget::index_changed);
 	connect(this->selectionModel(), &QItemSelectionModel::currentRowChanged,
-			this, &HistoryWidget::currentRowChanged);
+			this, &HistoryWidget::current_row_changed);
 }
 
-void HistoryWidget::indexChanged(int i)
+void HistoryWidget::index_changed(int i)
 {
 	this->selectionModel()->select(this->model()->index(i - 1, 0),
 								   QItemSelectionModel::ClearAndSelect |
 									   QItemSelectionModel::Rows);
 }
 
-void HistoryWidget::currentRowChanged(const QModelIndex &current,
+void HistoryWidget::current_row_changed(const QModelIndex &current,
 									  const QModelIndex &previous)
 {
 	size_t jump_to = (current.row() + 1);