Some improvements : gallery preview and gallery cache

[Jisay]

I allow myself to propose two other improvements (if not already done) :

• display some pictures as gallery preview instead of simply the name of the gallery with a color background. Like in the nextcloud gallery app.
• create a kind of cache instead of recreating images galleries. I don't know how it's done but maybe you can use the way it's done in the gallery app, the loading of a gallery is pretty fast the second time we access it.