Keep depth over navigation

[kelson42]

We need to keep a few parameter of the navigation (like the level of depth) chosen by the user over sessions. For that cookies are not supported, we have to use the local storage.

[benoit74]

We will store center and zoom, so that when user reload the map it stays where it was last time he used it.

This will be stored in local storage under openzim$${zimName}$default_view key:

• openzim prefix to minimize risk of conflict with other keys stored in local storage
• ${zimName}: replaced by the ZIM name, allowing to keep a setting per ZIM name: no conflict between various ZIMs which might contain different data ; no new setting everytime the ZIM/title is updated
• default_view: first setting we wanna store

Sample value: {"center":[8.997348031574575,41.99529678886276],"zoom":6.958897341884629}

Update frequency: everytime the maps moves, with a debounce.