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/common/debug.cpp

@@ -24,10 +24,10 @@
 namespace olive
 {
 
-void DebugHandler(QtMsgType type, const QMessageLogContext &context,
+void debug_handler(QtMsgType type, const QMessageLogContext &context,
 				  const QString &msg)
 {
-	QByteArray localMsg = msg.toLocal8Bit();
+	QByteArray local_msg = msg.toLocal8Bit();
 
 	const char *msg_type = "UNKNOWN";
 	switch (type) {
@@ -49,7 +49,7 @@ void DebugHandler(QtMsgType type, const QMessageLogContext &context,
 	}
 
 	//fprintf(stderr, "[%s] %s (%s:%u)\n", msg_type, localMsg.constData(), context.function, context.line);
-	fprintf(stderr, "[%s] %s\n", msg_type, localMsg.constData());
+	fprintf(stderr, "[%s] %s\n", msg_type, local_msg.constData());
 
 #ifdef Q_OS_WINDOWS
 	// Windows still seems to buffer stderr and we want to see debug messages immediately, so here we make sure each line