currently I am using the cordova-camera-plugin and I wonder if I can add a HTML sidebar to it which appears when I click a button?
Is it at all possible to add some buttons to the camera module?
And can I prevent the camera of asking me whether I want to keep the image or not?
Or should I maybe use another plugin?
You'd have to significantly modify the camera plugin to work like you're asking. You could search the plugin repository for some (seems to me like there's one or two out there that might work), OR, use the Media Capture API (if you're targeting a recent OS version) and provide your own custom UI. See http://caniuse.com/#feat=html-media-capture for support.