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/filefield/filefield.cpp

@@ -41,19 +41,19 @@ FileField::FileField(QWidget *parent)
 
 	line_edit_ = new QLineEdit();
 	connect(line_edit_, &QLineEdit::textChanged, this,
-			&FileField::LineEditChanged);
+			&FileField::line_edit_changed);
 	connect(line_edit_, &QLineEdit::textEdited, this,
-			&FileField::FilenameChanged);
+			&FileField::filename_changed);
 	layout->addWidget(line_edit_);
 
 	browse_btn_ = new QPushButton();
-	browse_btn_->setIcon(icon::Open);
+	browse_btn_->setIcon(icon::open);
 	connect(browse_btn_, &QPushButton::clicked, this,
-			&FileField::BrowseBtnClicked);
+			&FileField::browse_btn_clicked);
 	layout->addWidget(browse_btn_);
 }
 
-void FileField::BrowseBtnClicked()
+void FileField::browse_btn_clicked()
 {
 	QString s;
 
@@ -85,11 +85,11 @@ void FileField::BrowseBtnClicked()
 
 	if (!s.isEmpty()) {
 		line_edit_->setText(s);
-		emit FilenameChanged(s);
+		emit filename_changed(s);
 	}
 }
 
-void FileField::LineEditChanged(const QString &text)
+void FileField::line_edit_changed(const QString &text)
 {
 	if (QFileInfo::exists(text) || text.isEmpty()) {
 		line_edit_->setStyleSheet(QString());