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/audio/audiolevelmeter.cpp

@@ -29,7 +29,7 @@ namespace olive
 {
 
 AudioLevelMeter::Stats
-AudioLevelMeter::AnalyzeSampleBuffer(const core::SampleBuffer &samples)
+AudioLevelMeter::analyze_sample_buffer(const core::SampleBuffer &samples)
 {
 	Stats stats;
 
@@ -63,9 +63,9 @@ AudioLevelMeter::AnalyzeSampleBuffer(const core::SampleBuffer &samples)
 
 		ChannelStats channel_stats;
 		channel_stats.peak_linear = peak;
-		channel_stats.peak_db = LinearToDb(peak);
+		channel_stats.peak_db = linear_to_db(peak);
 		channel_stats.rms_linear = rms;
-		channel_stats.rms_db = LinearToDb(rms);
+		channel_stats.rms_db = linear_to_db(rms);
 		channel_stats.vu_db = channel_stats.rms_db;
 		stats.channels[channel] = channel_stats;
 
@@ -76,24 +76,24 @@ AudioLevelMeter::AnalyzeSampleBuffer(const core::SampleBuffer &samples)
 
 	stats.silence = qFuzzyIsNull(stats.max_peak_linear);
 	stats.integrated_lufs =
-		PowerToLufs(total_square / static_cast<double>(total_samples));
+		power_to_lufs(total_square / static_cast<double>(total_samples));
 
 	return stats;
 }
 
-double AudioLevelMeter::LinearToDb(double linear)
+double AudioLevelMeter::linear_to_db(double linear)
 {
 	if (linear <= 0.0) {
-		return Decibel::MINIMUM;
+		return Decibel::minimum;
 	}
 
-	return Decibel::fromLinear(linear);
+	return Decibel::from_linear(linear);
 }
 
-double AudioLevelMeter::PowerToLufs(double mean_square)
+double AudioLevelMeter::power_to_lufs(double mean_square)
 {
 	if (mean_square <= 0.0) {
-		return Decibel::MINIMUM;
+		return Decibel::minimum;
 	}
 
 	// BS.1770 loudness uses K-weighted mean square. This first pass stores the