Cancel any pending background operation when the applet frame is
destroyed, and avoid accessing the frame in the callback if the
operation was cancelled or failed.

Fixes #1557

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.

Fixes #1338: The clock-time was jumping (and also the rest of the panel), due to different width of the digits.

The font must support this feature, otherwise this fix will not work:
Example: Font "Cantarell" OK ; Font "Quicksand" still buggy.

The calendar popup was not freeing the list of events returned by the
calendar client after populating the appointment and task views. This
caused unbounded memory growth every time the popup was opened.

remove transifex configuration
remove "es_ES" as it is the same as "es" and prevent weblate usage
prevent po updates when generating pot files (weblate does that for us)