Allow to change mode from Chrome's toolbar button too

[adrienjoly]

Antonio was confused on how to change the current mode: he clicked on NS4Trello's extension button (in Chrome's toolbar)

...instead of on the Trello toolbar's button.

=> Suggested solution: clicking on NS4Trello's extension button (in browser toolbar) to behave according to context:

• if the user clicks it while the current tab is not a Trello board, show the same dialog as usual
• if the user clicks it while the current tab is a Trello board but NS4Trello is not injected in it yet, refresh the page after asking for permission (in a modal dialog)
• if the user clicks it while the current tab is a Trello board and NS4Trello was injected in it, display the options in the page, like if the user had clicked on NS4Trello's Trello toolbar button (see screenshot below)

... while making sure that the extension can still work when imported as a "user script" in browsers that don't support Chrome extensions. (=> we may need to create an automated test to prevent regressions, btw --> #108)

[adrienjoly]

For reference, here's the contents (e.g. mode selection options) of the "Next steps" button accessible thru Trello's toolbar: