Led matrix painter fixes and improvements

[BeanRepo]

Fixes:

• Add sanitizer method AppFrame._sanitize_c_ident() to ensure frame names C++ compliance
• Change data type in AppFrame.to_c_string() from uint32_t to uint_8_t
• Add missing argument for frame renaming that was causing an error on frame update

Improvements:

• Add animation exporters into Appframe to better handle animation export to sketch and to export.
• Unify UI/backend/sketch to 3-bit grayscale (0..7) format for Frames to match default matrix bit grayscale setting according to Arduino_LED_Matrix.h
• Add value normalization for frames stored in 0-255 value range for retrocompatibility with the previous version of the example.
• Add stop_animation endpoint and sketch logic to stop a playing animation