Pyxel

- Added the .pyxel file to the install example script

- Fixed the piano keyboard bug when only enter was pressed
- Fixed the piano keyboard highlights correctly
- Changed the default sound volume to 7
- Changed the default sound speed to 30
- Fixed the sound button bug of Sound Editor

- Changed the operation method of Sound Editor
- Changed to allow sound of length 0
- Implemented Music Editor
- Changed Example 2 to use resource file

- Implemented the undo function of Sound Editor
- Changed the click tolerance time
- Removed the length limitation of the sound
- Added the music and playm APIs
- Changed Example 2 to use the music and playm APIs

- Changed to follow the mouse position outside the window
- Changed to draw the self mouse cursor
- Added the mouse API
- Renamed arguments of the blt and bltm APIs
- Reduced the size of PNG and animated GIF
- Changed the max length of the sound to 48
- Added the system option to the sound API
- Refined the help message of Pyxel Editor
- Added the ToggleButton and ImageToggleButton
- Implemented Sound Editor except the undo function

- Fixed the right click bug in Tilemap Editor
- Fixed the key callback bug
- Added the get method to the Image class
- Changed the set method of the Image class to accept a number as data
- Added the get and set methods to the Tilemap class
- Added the bltm test to Example 3
- Updated the descriptions of the READMEs