scratch: Use an explicit marker for floating windows

On startup we use a heuristic to figure out which windows should float in the
scratch layer, but from then on we simply check if the window is marked as
floating.

NB: Since `above` is reset when a window is minimized there's no guarantee that
a floating window will be above when visible.

scratch.js

@@ -2,13 +2,16 @@ const Extension = imports.misc.extensionUtils.extensions['paperwm@hedning:matrix
 const Meta = imports.gi.Meta;
 const utils = Extension.imports.utils;
 const debug = utils.debug;
+let float;
 
 function makeScratch(metaWindow) {
+    metaWindow[float] = true;
     metaWindow.make_above();
     metaWindow.stick();
 }
 
 function unmakeScratch(metaWindow) {
+    metaWindow[float] = false;
     metaWindow.unmake_above();
     metaWindow.unstick();
 }
@@ -22,7 +25,7 @@ function toggle(metaWindow) {
 }
 
 function isScratchWindow(metaWindow) {
-    return metaWindow && (metaWindow.above || metaWindow.minimized);
+    return metaWindow[float];
 }
 
 /** Return scratch windows in MRU order */
@@ -60,6 +63,10 @@ const PopupMenu = imports.ui.popupMenu;
 const WindowMenu = imports.ui.windowMenu;
 const originalBuildMenu = WindowMenu.WindowMenu.prototype._buildMenu;
 
+function init() {
+    float = Symbol();
+}
+
 function enable() {
     WindowMenu.WindowMenu.prototype._buildMenu =
         function (window) {

tiling.js

@@ -831,6 +831,12 @@ function add_all_from_workspace(workspace) {
     let space = spaces.spaceOf(workspace);
     windows.forEach((meta_window, i) => {
         if(space.indexOf(meta_window) < 0 && add_filter(meta_window, true)) {
+            if (meta_window.above || meta_window.minimized) {
+                // Rough heuristic to figure out if a window should float
+                Scratch.makeScratch(meta_window);
+                return;
+            }
+
             // Using add_handler is unreliable since it interacts with focus.
             space.push(meta_window);
             meta_window[focus_signal] = meta_window.connect("focus", focus_wrapper);