Internal files reorganisation

Improved files navigation by using simple folders
* lib/      - Javascript libraries
* module/   - Internal modules
* doc/      - Forum bbcode posts and later full dev doc
* tools/    - Powershell scripts (extensions auto-build)
* images/*/ - Images themes
Updating code to reflect folders change - Themes

Mostly on images folders, since theming was added
Added a theme selector in options with appropriate save
management
Theses changes won't directly update on css, page refresh
is needed
Images for 2nd theme are missing
Untested
Going further in files refactoring

Moved all general functions into lib/gksi_utils.js under
utils.foo() name
The only functions left in gks_improved.js are affecting
the DOM and CSS stuff
How about moving them to the global module ?
Options also moved in their own file
gks_improved.js is now less clutered and easier to understand