mobile: API for nav drawer

Allow plugins to define new panes for the navigation drawer. This patch includes a naive sample implementation for the bookmarks plugin.

@ZasoGD, please have a look at this, I'm not sure whether I did right. If necessary, undo commit 8a10848e15fc3dd2057a6582b038db00846bebe1 and do it the right way.

@leCradle: please don't merge this pr until @ZasoGD has reviewed the bookmarks plugin. I don't want the bookmarks plugin to be broken on desktop.