Add dont-steal-focus configuration property.

Some applications send _NET_ACTIVE_WINDOW to activate their window. EXWM
respects this message always and brings such up into focus. This might
be unintended and there should be a way to turn this off. In my case the
app is Spotify desktop client installed from Flatpak.

Fixes ch11ng/exwm#732

exwm-manage.el

@@ -65,6 +65,7 @@ possible choices:
 * prefix-keys: `exwm-input-prefix-keys' local to this X window.
 * simulation-keys: `exwm-input-simulation-keys' local to this X window.
 * workspace: The initial workspace.
+* dont-steal-focus: Ignore _NET_ACTIVE_WINDOW signal from this X window.
 * managed: Force to manage (non-nil) or not manage (nil) the X window.
 
 For each X window managed for the first time, matching criteria (sexps) are
@@ -98,6 +99,7 @@ want to match against EXWM internal variables such as `exwm-title',
                          (alist :key-type (key-sequence :tag "From")
                                 :value-type (key-sequence :tag "To")))
                         ((const :tag "Workspace" workspace) integer)
+                        ((const :tag "Don't steal focus" dont-steal-focus) boolean)
                         ((const :tag "Managed" managed) boolean)
                         ;; For forward compatibility.
                         ((other) sexp))))

exwm.el

@@ -494,7 +494,8 @@ DATA contains unmarshalled PropertyNotify event data."
     (if (buffer-live-p buffer)
         ;; Either an `exwm-mode' buffer (an X window) or a floating frame.
         (with-current-buffer buffer
-          (when (eq exwm--frame exwm-workspace--current)
+          (when (and (eq exwm--frame exwm-workspace--current)
+                     (not (plist-get exwm--configurations 'dont-steal-focus)))
             (if exwm--floating-frame
                 (select-frame exwm--floating-frame)
               (setq window (get-buffer-window nil t))