Release 4

Main changes since the last release:

• Added multi_disk_image SD card build system
• Debounce only the release of the UPDATE button
• Fix some minor audio issues (clicks and pops when switching cores and resetting)
• Electron: Update BREAK key to F12 to match other cores
• Electron: Fix vsync timing issue
• Electron: Workaround incorrect optimization of dac1_oser VGA DAC
• Electron: Recenter Mode 7 image

Full Changelog: release_3...release_4