add binding file and work with it (without default hardcode binding)
Merge remote-tracking branch 'mike/master'
make default binding a fall-back binding
avoid some invalid keybinding pattern, and save it back to config file

1. empty string
2. invalid key
3. remove keybinding
add F10 as default key binding to open keybinding setting file
update Help.txt to add F10 key binding
Fix bug when opening recent file dialog first time without any recent file already.
Merge branch 'master' of https://github.com/rogercloud/Markdown-Edit into rogercloud-master
Change to only load once, don't write file on every program start.
Refactor
More refactoriing