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/taskview/taskview.cpp

@@ -45,21 +45,21 @@ TaskView::TaskView(QWidget *parent)
 	layout_->addStretch();
 }
 
-void TaskView::AddTask(Task *t)
+void TaskView::add_task(Task *t)
 {
 	// Create TaskViewItem (UI representation of a Task) and connect it
 	TaskViewItem *item = new TaskViewItem(t);
-	connect(item, &TaskViewItem::TaskCancelled, this, &TaskView::TaskCancelled);
+	connect(item, &TaskViewItem::task_cancelled, this, &TaskView::task_cancelled);
 	items_.insert(t, item);
 	layout_->insertWidget(layout_->count() - 1, item);
 }
 
-void TaskView::TaskFailed(Task *t)
+void TaskView::task_failed(Task *t)
 {
-	items_.value(t)->Failed();
+	items_.value(t)->failed();
 }
 
-void TaskView::RemoveTask(Task *t)
+void TaskView::remove_task(Task *t)
 {
 	items_.value(t)->deleteLater();
 	items_.remove(t);