Refactor Main.js

Main.js is too big to modify now .  Add new file (TodoContext.js) to
maintain extension context and isolate modification of brackets API.
make settings dialog UI
Implement settings dialog
add tooltip for settings dialog's input fields
init settings dialog values using .todo file settings or default settings at first time
Set indentation to 8 spaces
Keep file visibility as before after file changed
Merge pull request #64 from anzhihun/master

Merge code for settings dialog.