update matching a MIME type accoding to the data URI format
fix wrong count resetting
refactor preference constants

* move the constant for visible lines of a cropped text to the global
  preference
* pack constants in one, |kPref|
refactor the function of making a tip data

* change a 'do not crop' parameter to 'do crop' for readability
* separate the line-wrapping function for readability
* store a wrapped whole text for a cropped text, |uncroppedText|. this will be
  tooltip text in the future
apply well-ordered newlines to a tooltip text for a cropped data
change the module format of the main handler from object to function

* I prefer it
simplify the check of whether a textbox is in the browser window

* It seems to have missed something error. I don't know much about a reliable
  checking
* it looks like working well in this simple setting