Restore keyboard grabbing after exiting fullscreen

Recover the difference between the input mode selected by the user and
actual one, which might be transient.  This was inadvertently removed in
recent changes.

* exwm-core.el (exwm--selected-input-mode): New variable.
* exwm-input.el (exwm-input-grab-keyboard)
(exwm-input-release-keyboard): Use it.
* exwm-layout.el (exwm-layout-unset-fullscreen): Return to the
selected input mode.

exwm-core.el

@@ -158,7 +158,11 @@ least SECS seconds later."
 (defvar-local exwm--mode-line-format nil) ;save mode-line-format
 (defvar-local exwm--floating-frame-position nil) ;set when hidden.
 (defvar-local exwm--fixed-size nil)              ;fixed size
-(defvar-local exwm--input-mode 'line-mode)       ;Keyboard grabbed.
+(defvar-local exwm--selected-input-mode 'line-mode
+  "Input mode as selected by the user.
+One of `line-mode' or `char-mode'.")
+(defvar-local exwm--input-mode 'line-mode
+  "Actual input mode, i.e. whether mouse and keyboard are grabbed.")
 ;; Properties
 (defvar-local exwm--desktop nil "_NET_WM_DESKTOP.")
 (defvar-local exwm-window-type nil "_NET_WM_WINDOW_TYPE.")

exwm-input.el

@@ -756,6 +756,7 @@ button event."
                        (exwm--buffer->id (window-buffer)))))
   (when id
     (exwm--log "id=#x%x" id)
+    (setq exwm--selected-input-mode 'line-mode)
     (exwm-input--grab-keyboard id)
     (exwm-input--update-mode-line id)))
 
@@ -766,6 +767,7 @@ button event."
                        (exwm--buffer->id (window-buffer)))))
   (when id
     (exwm--log "id=#x%x" id)
+    (setq exwm--selected-input-mode  'char-mode)
     (exwm-input--release-keyboard id)
     (exwm-input--update-mode-line id)))
 

exwm-layout.el

@@ -199,7 +199,7 @@
         (make-instance 'xcb:ewmh:set-_NET_WM_STATE :window exwm--id :data []))
     (xcb:flush exwm--connection)
     (set-window-dedicated-p (get-buffer-window) nil)
-    (when (eq 'line-mode exwm--input-mode)
+    (when (eq 'line-mode exwm--selected-input-mode)
       (exwm-input--grab-keyboard exwm--id))))
 
 ;;;###autoload