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/task/customcache/customcachetask.cpp

@@ -27,10 +27,10 @@ namespace olive
 CustomCacheTask::CustomCacheTask(const QString &sequence_name)
 	: cancelled_through_finish_(false)
 {
-	SetTitle(tr("Caching custom range for \"%1\"").arg(sequence_name));
+	set_title(tr("Caching custom range for \"%1\"").arg(sequence_name));
 }
 
-void CustomCacheTask::Finish()
+void CustomCacheTask::finish()
 {
 	mutex_.lock();
 
@@ -40,11 +40,11 @@ void CustomCacheTask::Finish()
 	mutex_.unlock();
 }
 
-bool CustomCacheTask::Run()
+bool CustomCacheTask::run()
 {
 	mutex_.lock();
 
-	while (!IsCancelled()) {
+	while (!is_cancelled()) {
 		wait_cond_.wait(&mutex_);
 	}
 
@@ -56,7 +56,7 @@ bool CustomCacheTask::Run()
 void CustomCacheTask::CancelEvent()
 {
 	if (!cancelled_through_finish_) {
-		emit Cancelled();
+		emit cancelled();
 	}
 	wait_cond_.wakeOne();
 }