Improvements for the image viewer

The first version of the image viewer has been implemented but there are several issues to fix / improvements that can be done :

• The buttons to zoom in and out are always sensitive: the zoom out button should be insensitive when the user is at the minimal zoom level and the zoom in button should be insensitive when the user is at the maximal zoom level.
• The image is not resized when entering in full screen mode.
• It is not possible to go back (with the previous button) beyond the loaded messages.
• There is no possibility for zooming to more than 100% of the zoom level.
• The horizontal scroll is not always available when an image is bigger than the view port.
• There should be 6px padding in the overflow menu popover.
• There should be a header bar available in full screen mode.
• Hide arrow buttons instead of making them insensitive, when there are no images in the direction
• In fullscreen mode, show headerbar when pushing the cursor against the top of the screen. In this headerbar, the fullscreen button is replaced with a non-fullscreen button, like in Builder
• Allow navigating the images using left/right keys