Commit 91333dacfa1, which adds the feature of optionally skipping over
inactive widgets when tabbing (bug#70413), fails to handle buffers that
contain only one active widget; in that case, pressing TAB or S-TAB
raises the error "No buttons or fields found".  To reproduce:

0. emacs -Q
1. Load the attached file widget-test.el and then type `M-x
   my-widget-test'.
2. In the buffer "*My Widget Test*" repeatedly press TAB and S-TAB and
   observe that point moves successively between the active radio
   buttons labelled "One" and "Two" and the active push button "Submit",
   but skips the inactive push button "Reset", since the user option
   widget-skip-inactive is enabled.
3. Now press (via RET or mouse-click) the "Submit" button, which makes
   the radio buttons and the "Submit" button inactive and the "Reset"
   button active.
4. Pressing TAB once moves point to the "Reset" button, then pressing
   TAB again moves point to BOB and raises the error "No buttons or
   fields found".  Likewise, pressing S-TAB once moves point to "Reset",
   then pressing S-TAB again moves to EOB and raises the same error.

The attached patched fixes this bug.  In addition, when only one widget
is active and point is on it, since pressing TAB or S-TAB does not move
point, the informational message "Only one tabable widget" is displayed.

The original code in widget-move contained separate loops to handle
forward and backward movement and this involved some code duplication.
The changes to this code to fix the bug would have required further
duplication, so I took the opportunity to parameterize the
directionality, allowing handling both directions in a single loop.
This shortens the code and IMO makes the similarities and differences in
the handling easier to see.


Configured using:
 'configure 'CFLAGS=-Og -g3' PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER
WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB