Scene: Ensure classes match their header filename

Also drop some unused files.

Renamed:
- `scene/2d/navigation2d.h` -> `navigation_2d.h`
- `scene/2d/screen_button.h` -> `touch_screen_button.h`
- `scene/3d/scenario_fx.h` -> `world_environment.h`
- `scene/audio/audio_player.h` -> `audio_stream_player.h`
- `scene/resources/bit_mask.h` -> `bit_map.h`
- `scene/resources/color_ramp.h` -> `gradient.h`
- `scene/resources/shape_line_2d.h` -> `line_shape_2d.h`
- `scene/resources/scene_format_text.h` -> `resource_format_text.h`
- `scene/resources/sky_box.h` -> `sky.h`

Dropped:
- `scene/resources/bounds.h`

editor/editor_export.cpp

@@ -42,7 +42,7 @@
 #include "editor/plugins/script_editor_plugin.h"
 #include "editor_node.h"
 #include "editor_settings.h"
-#include "scene/resources/scene_format_text.h"
+#include "scene/resources/resource_format_text.h"
 #include "thirdparty/misc/md5.h"
 
 static int _get_pad(int p_alignment, int p_n) {