REDESIGNED: `LoadDirectoryFiles()`
ADDED: `LoadDirectoryFilesEx()`
REDESIGNED: `LoadDroppedFiles()`
ADDED: `IsPathFile()`

This BIG BREAKING change simplifies the functions and gives more control to the user:
 - A new `struct FilePathList` has been added to avoid exposing complex pointers.
 - User is responsible of memory loading/unloading
 - Filepaths loading support recursive directories and file extension filters

RENAMED: `ClearDroppedFiles()` to `UnloadDroppedFiles()`

Renamed for consistency with other function in raylib.

* Added support for additional mouse buttons

* Renamed mouse button enum

Co-authored-by: Lambert Wang <lambert.ww@gmail.com>

Some enums values have been renamed to be more consistent and also provide a more detailed description:
 - ShaderLocationIndex:    LOC_VERTEX_POSITION -> SHADER_SHADER_LOC_VERTEX_POSITION
 - ShaderUniformDataType:  UNIFORM_VEC2 -> SHADER_UNIFORM_VEC2
 - MaterialMapType: MAP_ALBEDO -> MATERIAL_MAP_ALBEDO
 - PixelFormat: UNCOMPRESSED_GRAYSCALE -> PIXELFORMAT_UNCOMPRESSED_GRAYSCALE

They are not automatically unloaded when unloading the model!

- Mesh, Shader, Material structs have been reviewed to minimize size when passed by value, all required code has been reviewed.
 - GetCollisionRayModel() reviewed to avoid pointer, not required because model is not modified inside the function
 - UnloadMesh() reviewed, pointer not required
 - CheckCollisionRay*() parameters name reviewed