Shell: update screenshot and screen recorder

gnome-shell allows taking screenshots and has a built-in screen recorder, it can take screenshots and record screencasts of an area, a window or the whole monitor. Right now, screenshots can be taken using the gnome-screenshot utility or by pressing certain key combinations, recording screencasts only works by pressing a key combination. Remembering all those combinations is hard, especially when not using them often, also it turned out a lot of people are not even aware features like the screen recorder exist. The plan for this project is to unify both the screenshot and the screen recorder functionality within a single UI that's part of gnome-shell. This will make it easy to select an area or a window to screenshot/to record, especially on devices like tablets or phones where no keyboard and mouse are available.