Implement MIDI I/O Device Opening Functionality

This PR implements actual MIDI device port opening functionality in the C++ MidiDeviceManager to achieve feature parity with the Kotlin implementation. Previously, the C++ version only stored device names without actually opening MIDI ports.

Changes Made

Core Implementation

• Added libremidi MIDI I/O members: Added std::unique_ptr<libremidi::midi_in> and std::unique_ptr<libremidi::midi_out> to MidiDeviceManager::Impl
• Implemented actual port opening: Updated set_input_device() and set_output_device() methods to use libremidi API for opening MIDI ports
• Added callback functionality: Implemented midi_input_opened_ and midi_output_opened_ callback lists matching the Kotlin pattern
• Added callback registration methods: Added add_input_opened_callback() and add_output_opened_callback() public methods

MIDI Input Handling

• Input callback setup: Configured libremidi::input_configuration with .on_message callback to process incoming MIDI messages
• SysEx processing: Connected MIDI input callback to existing process_incoming_sysex() method for message dispatch
• Proper configuration: Set .ignore_sysex = false to ensure SysEx messages are received

MIDI Output Enhancement

• Enhanced send_sysex(): Updated to use the opened MIDI output port when available, falling back to custom sender
• Error handling: Added proper error handling with libremidi's stdx::error system

Resource Management

• Proper cleanup: Updated shutdown() method to close MIDI ports before shutdown
• Port lifecycle: Implemented proper opening/closing of ports when devices are changed

Build System

• Optional Qt5: Made Qt5 tool optional in CMake configuration to avoid build issues when Qt5 is not available

Technical Details

libremidi API Integration

• Uses libremidi::observer to enumerate available MIDI ports
• Creates libremidi::midi_in with configuration-based callback setup
• Creates libremidi::midi_out for sending MIDI messages
• Proper error handling with stdx::error and stdx::string_ref conversion

Callback Pattern Matching

The implementation follows the exact pattern from the Kotlin version:

• Kotlin: midiInputOpened and midiOutputOpened callback lists
• C++: midi_input_opened_ and midi_output_opened_ callback vectors
• Both invoke registered callbacks when ports are successfully opened

Error Handling

• Comprehensive error handling for port opening failures
• Proper conversion of stdx::string_ref to std::string for error messages
• Graceful fallback when port opening fails

Testing

• ✅ Compilation: Successfully compiles with libremidi integration
• ✅ Build system: CMake configuration works with optional Qt5
• ✅ API compatibility: Maintains existing public API while adding new functionality

Compatibility

This change is backward compatible:

• Existing API methods unchanged
• New callback methods are optional additions
• Fallback behavior maintained for send_sysex when no output port is open

Link to Devin run

https://app.devin.ai/sessions/ad9744cfe3c64558ae6f5adb9fadf986

Requested by: Atsushi Eno (atsushieno@gmail.com)