Add Volume Control

[Totto16]

This adds a Settings Menu and a rudimentary Slider to control the volume
It also adds some more keyboard shortcuts, notably:

• "+" / "-" for general volume control
• "P" for opening the settings while playing

This also fixes some uncaught logic bugs in the music manager with the fading in and out logic

Some screenshots:

[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-format reports: 1 file(s) not formatted

• src/capabilities.hpp

clang-tidy reports: 11 concern(s)

• src/application.cpp

  src/application.cpp:69:11: warning: [readability-else-after-return]

  do not use 'else' after 'return'

          } else if (event.key.keysym.sym == SDLK_MINUS or event.key.keysym.sym == SDLK_KP_MINUS) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• src/music_manager.hpp

  src/music_manager.hpp:18:25: warning: [cppcoreguidelines-avoid-non-const-global-variables]

  variable 'step_width' is non-const and globally accessible, consider making it const

      static inline float step_width = 0.05F;
                          ^
  /home/runner/work/oopetris/oopetris/src/music_manager.hpp:18:38: warning: 0.05F is a magic number; consider replacing it with a named constant [cppcoreguidelines-avoid-magic-numbers,readability-magic-numbers]
      static inline float step_width = 0.05F;
                                       ^

  src/music_manager.hpp:44:21: warning: [readability-avoid-const-params-in-decls]

  parameter 'new_volume' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions

      void set_volume(const tl::optional<float> new_volume, const bool force_update = false);
                      ^~~~~~

  src/music_manager.hpp:44:59: warning: [readability-avoid-const-params-in-decls]

  parameter 'force_update' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions

      void set_volume(const tl::optional<float> new_volume, const bool force_update = false);
                                                            ^~~~~~

  src/music_manager.hpp:46:39: warning: [readability-avoid-const-params-in-decls]

  parameter 'steps' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions

      tl::optional<float> change_volume(const std::int8_t steps);
                                        ^~~~~~

• src/scenes/scene.cpp

  src/scenes/scene.cpp:8:26: warning: [readability-identifier-length]

  parameter name 'id' is too short, expected at least 3 characters

      Scene::Scene(SceneId id, ServiceProvider* service_provider) : m_id{ id }, m_service_provider{ service_provider } { }
                           ^

• src/scenes/scene.hpp

  src/scenes/scene.hpp:45:32: warning: [readability-identifier-length]

  parameter name 'id' is too short, expected at least 3 characters

          explicit Scene(SceneId id, ServiceProvider* service_provider);
                                 ^

• src/ui/slider.hpp

  src/ui/slider.hpp:47:17: warning: [modernize-pass-by-value]

  pass by value and use std::move

                  const Range& range,
                  ^~~~~~~~~~~~
                  Range 

  src/ui/slider.hpp:48:17: warning: [modernize-pass-by-value]

  pass by value and use std::move

                  const Getter& getter,
                  ^~~~~~~~~~~~~
                  Getter 

  src/ui/slider.hpp:49:17: warning: [modernize-pass-by-value]

  pass by value and use std::move

                  const Setter& setter,
                  ^~~~~~~~~~~~~
                  Setter 

• src/utils.hpp

  /home/runner/work/oopetris/src/utils.hpp:89:9: warning: [cppcoreguidelines-macro-usage]

  function-like macro 'UNUSED' used; consider a 'constexpr' template function

  #define UNUSED(x) (void(x))
          ^

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