The window-list preview code uses XGetWindowAttributes and
XGetWindowProperty to capture window decorations, but the wncklet
Makefile was missing the X11 linker flags, causing a build failure.

Fixes #1561

Account for cairo device scale factor when scaling the captured pixbuf
to the thumbnail surface, so the entire window content is shown on HiDPI
displays.

Use gdk_pixbuf_get_from_window instead of gdk_cairo_set_source_window to
avoid stale window content in preview thumbnails.

Capture the window manager frame instead of the client window so that
preview thumbnails include title bars and borders. Use
_NET_FRAME_EXTENTS to strip invisible resize borders from the captured
frame.

*Note that in the wayland panel the show desktop applet works by using an invocation of the window list backend to send a signal to every window to mimimize or unminimize
*This however causes every added button to be counted twice when the show desktop applet is also used
*Detect the show desktop applet, and only divide the number of window list buttons by 2 when it is present

The monitor origin needs to be taken into account.

*Set a sane default width of 1/3 the total panel with on startup
*This prevents the panel width from coming up zero or one when GTK gets the allocation with no buttons in it

*Always affirmatively show or hide icons and text when adjusting the buttons for width
*Get the box the tasklist->outer box is packed into for width calculations
*Recompute tasklist width both on adding and removing buttons

*Recompute the maximum button width when three fullsize buttons won't fit
*Otherwise short tasklists overflow