Typing performance improvement

I discovered this when implementing editor preference validation. For `EditorStatusBar` to display the cursor position, it calls `Editor.getCursorPos()` with `expandTabs` option. This causes the "tabSize" preference to be retrieved on *every* keystroke. This really sucks in a document that doesn't have any Tab characters!

This changes it so the "tabSize" pref is not retrieved until a until a Tab char is found.

This value could be cached to improve it for docs with Tab chars since it rarely changes.