add a swapping button to swap between languages

* Basic changes in functionality to improve performance
* Nicer UI and displaying more details on translations such as: Definitions and Examples
* Choose <Ctrl> + <Shift> + as the hotkey to enable and disable translation
* Arrow up and down keys for scrolling translations
* a mechanism to alarm users the new version exists
* and more

1- some improvement in the style
2- an enable/disable button added to toolbar
3- we have some changes in the functionality
4- the icons were changed

change in v1.2.2:

a bug in styles.css was fixed.

Major changes in v1.2.0

1- displaying phonetic
2- help-cursor on the addon's enable mode
3 - displaying translation text and the source text in the modal
4- add listen button to the modal
5- translate synonyms by clicking on them
6- improvement in the styles

major changes:

1- add a translation form to translate from one language to other in the modal.
2- prefernce page changed. two select options added in the page to choose source language and target language.
3- while network is not accissible, a message in the modal appears to inform users.
4- easily close translation modal with click on the backdrop modal.
5- new preference page for add-on.