Board Revision Update

Advanced to .Net Framework 4.8 (from 4.6.1).
Changed the board revision to match the machine:

• EMB: Rev B
• EMC: Rev C
• EMU: Rev U
• EM+: Rev U+

Added loading of Tilemaps (*.tlm) in Asset Loader.
Fixed the annoying issue when saving Palette files from Asset Window.
Added the Restart from Default Kernel command to the Reset menu.
When using command-line switches, if the autorun parameter is not specified, read the context file.
Updated the README file with keyboard shortcuts.