Cache the Motif drag window to avoid fetching it every time

* src/xterm.c (xm_get_drag_window_1): New function.
(xm_get_drag_window): Cache the window.  If it already exists,
just return it.
(xm_setup_dnd_targets): If a BadWindow error occurs, re-create
the Motif drag window.

* src/xterm.h (struct x_display_info): New field
`motif_drag_window'.
2 jobs for master in 47 minutes and 50 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Images
passed #47797
build-image-inotify

00:45:41

 
  Normal
failed #47798
test-src-inotify

00:02:09

 
Name Stage Failure
failed
test-src-inotify Normal There has been a runner system failure, please try again
ERROR: Preparation failed: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running? (<autogenerated>:162:120s)
Will be retried in 3s ...
Using Docker executor with image docker:19.03.12 ...
ERROR: Preparation failed: Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-cache-3c3f060a0374fc8bc39395164f415a70" is already in use by container "644ad5125bbbb9cbb5e3c26dc68ced4af4f5dbc6e1bf7a1691853df3c3255446". You have to remove (or rename) that container to be able to reuse that name. (<autogenerated>:162:0s)
Will be retried in 3s ...
Using Docker executor with image docker:19.03.12 ...
ERROR: Preparation failed: Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-cache-3c3f060a0374fc8bc39395164f415a70" is already in use by container "644ad5125bbbb9cbb5e3c26dc68ced4af4f5dbc6e1bf7a1691853df3c3255446". You have to remove (or rename) that container to be able to reuse that name. (<autogenerated>:162:0s)
Will be retried in 3s ...
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-cache-3c3f060a0374fc8bc39395164f415a70" is already in use by container "644ad5125bbbb9cbb5e3c26dc68ced4af4f5dbc6e1bf7a1691853df3c3255446". You have to remove (or rename) that container to be able to reuse that name. (<autogenerated>:162:0s)