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/pluginSupport/paraminstance.cpp

@@ -19,13 +19,13 @@
 
 #include "paraminstance.h"
 
-#include "OlivePluginInstance.h"
+#include "oliveplugininstance.h"
 
 namespace olive
 {
 namespace plugin
 {
-void SubmitUndoCommand(const std::shared_ptr<PluginNode> &node,
+void submit_undo_command(const std::shared_ptr<PluginNode> &node,
 					   UndoCommand *command, const QString &label)
 {
 	if (!command) {
@@ -36,12 +36,12 @@ void SubmitUndoCommand(const std::shared_ptr<PluginNode> &node,
 		auto *instance = node->getPluginInstance();
 		auto *olive_instance = dynamic_cast<OlivePluginInstance *>(instance);
 		if (olive_instance) {
-			olive_instance->SubmitUndoCommand(command, label);
+			olive_instance->submit_undo_command(command, label);
 			return;
 		}
 	}
 
-	if (!IsGuiThread()) {
+	if (!is_gui_thread()) {
 		command->redo_now();
 		delete command;
 		return;