Bookmark::fromArray does not save custom data set by plugins on save_link

[Kdecherf]

The new Bookmark object and its service introduced in v0.12 remove custom data set by save_link plugin hooks.

This behavior has two drawbacks:

1. All custom data set when saving new links are lost (_e.g. with the provided demoplugin)
2. All custom data set on links saved with previous versions of Shaarli are lost when updateMethodMigrateDatabase is executed

Bookmark::fromArray() should support a custom key to let plugins add again custom data.

Example of plugins saving custom data on links: via

[nodiscc]

Duplicate with a bit more information: https://github.com/shaarli/Shaarli/issues/2000