Ensure that such use cases as forced-csd environments don't cause the desktop window to be decorated

*Do not use G_OBJECT_CLASS (class)->constructed
*We don't seem to need it anymore and it breaks dragging windows with the titlebar or resizing with the mouse in wayland

* Wayland-background-dialog: fix memory leaks

* Wayland-background-dialog: check that the image is valid before using it

*Reduce the scope of the preview box

*Do not read the same value from gsettings before reading it from the color style combobox

Move image and colors labels, filebutton to left side of dialog

*Draw the background on the desktop window, using the same code as on navigation windows to bypass the no-root-window issue.
*Works from caja's "Change Desktop Background" menu item, from caja-extensions "set as wallpaper" and with changing the background in dconf-editor.
*All background options available in x11 supported in wayland, UI is new for wayland

*Apply changes immediately, keep the dialog open until closed by user
*Let users see changes in realtime while keeping the dialog open
*Preview both colors and pictures
*Add ability to apply changes from either combobox including using any image saved from when dialog was opened
*Add tooltips to the buttons, filepicker, and comboboxes