TabBox: Avoid unnecesary resets of the client model
When the tabbox switcher is shown and any window is added or removed, its client model is fully reset, even if this window is not included in the model. This can be a bit expensive and also produce small visual quirks on certain switchers, so let's check if the window list changes before resetting the model. BUG: 466660 FIXED-IN: 5.27.3 (cherry picked from commit e89c09a6)