Replace deprecated Nett library with Tomlyn

• fix treeview checkbox handling
• load alternative toml files
• updated portaudio to 19.7

No new features, just bumped .net Version to 6.0

fix for unicode device names (cyrillic etc) (thanks to https://github.com/vasilevp)

No changes, just switched to .net 5.0

• Fixes issue where BufferSize was always null
• Adds link to FlexASIO docs
• Adds function to refresh device list while GUI is open

• adds a Registry key with the install path to HKEY_LOCAL_MACHINE\SOFTWARE\Fabrikat\FlexASIOGUI\Install\Installpath
• fixes issue where (None) was written to config file instead of empty string (#14)
• makes most settings optional so that defaults are kept (#11)

Should fix non-ascii name issues #7

First version. Use at own risk...