lib/model, gui: Allow creating and editing ignores of paused folders (fixes #3608)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3996
LGTM: calmh, AudriusButkevicius

lib/model/folder.go

@@ -10,9 +10,11 @@ import "time"
 
 type folder struct {
 	stateTracker
-	scan  folderScanner
-	model *Model
-	stop  chan struct{}
+
+	scan                 folderScanner
+	model                *Model
+	stop                 chan struct{}
+	initialScanCompleted chan struct{}
 }
 
 func (f *folder) IndexUpdated() {
@@ -23,6 +25,7 @@ func (f *folder) DelayScan(next time.Duration) {
 }
 
 func (f *folder) Scan(subdirs []string) error {
+	<-f.initialScanCompleted
 	return f.scan.Scan(subdirs)
 }
 func (f *folder) Stop() {