Add a property to control assets root directory

[voxelias]

1. Adds the root property to Assets service, that defaults to current working directory
2. Adds method root() to Assets to get value of the property
3. Adds method set_root() to Assets to set value of the property
4. Adds method import_from to Assets to keep possibility to import file from anywhere

For iOS applications some startup system should be considered, for example:

fn startup(mut assets: Mut<Assets>) {
  // before loading any asset from FS
  assets.set_root(
    std::env::current_exe()
      .expect("Executable path to be available")
      .parent()
      .unwrap()
  );
}

[voxelias]

Based on the PR #145 from @russellwmy