Enhance the saving tiles feature

[dtrucs]

In order to display map tiles when the user is offline, there is a mechanism for storing them in the client's browser.
Today, this features presents two problems:

• Tiles are saved immediately when the page is loaded (instead of waiting for the user's explicit action to save the page for offline use).
• Tiles are stored progressively as the user manipulates the map, which means that if the user doesn't touch the map before going offline, only tiles visible at screen size are saved. In this user story, geometry with large boundaries may not have its entire lineString covered by tiles.