DesktopConfiguration: shared mode

This teaches the DesktopConfiguration command about a new option:
shared.  When in this mode, a shared state means that's there one set of
desktops (defined via the `DesktopName` command), but these desks are
global to all monitors, yet monitors may independently selected which
desktop to show at that time.

This command will only work if the number of monitors active at the time
`DesktopConfiguration shared` was set is >1, and the number of desktops
set (via `DesktopName` command) is >1.

If a request to move to a particular desktop matches one already shown
on a different monitors, these two desks are exchanged between those
monitors.

Fixes #260

fvwm/placement.c

@@ -340,7 +340,8 @@ adjust_for_shared_placement(FvwmWindow *fw, const exec_context_t *exc)
 	if (DO_NOT_SHOW_ON_MAP(fw))
 		goto done;
 
-	if (SUSE_START_ON_SCREEN(&style.flags)) {
+	if (SUSE_START_ON_SCREEN(&style.flags) ||
+	    SUSE_START_ON_DESK(&style.flags)) {
 		struct monitor	*m_style = NULL;
 		char		*sc = SGET_START_SCREEN(style), *e = NULL;
 
@@ -349,6 +350,18 @@ adjust_for_shared_placement(FvwmWindow *fw, const exec_context_t *exc)
 		if (e != NULL)
 			m_style = monitor_resolve_name(e);
 
+		/* We might not have ascertained the screen yet -- especially
+		 * if only StartsOnDesk was used.  So either we can use the
+		 * screen the window is on, or use the currently active one if
+		 * not.
+		 */
+		if (m_style == NULL) {
+			if (fw->m != NULL)
+				m_style = fw->m;
+			else
+				m_style = monitor_get_current();
+		}
+
 		xasprintf(&cmd, "GotoDesk %s 0 %d", m_style->si->name,
 				fw->Desk);
 		execute_function_override_window(NULL, NULL, cmd, 0, fw);

fvwm/virtual.c

@@ -2600,6 +2600,7 @@ void CMD_GotoDeskAndPage(F_CMD_ARGS)
 		CMD_GOTO_DESK(m, val[0]);
 	} else 	if (is_new_desk) {
 		UnmapDesk(m, m->virtual_scr.CurrentDesk, True);
+		CMD_GOTO_DESK(m, val[0]);
 	}
 	m->virtual_scr.prev_desk_and_page_page_x = m->virtual_scr.Vx;
 	m->virtual_scr.prev_desk_and_page_page_y = m->virtual_scr.Vy;