feat: 合并 henkman#20 (切换桌面时记住焦点窗口)

Title: remember focused window when switching desktops
Links: henkman#20

```Describe-Text
i was a bit annoyed that switching desktops would sometimes keep the hidden window focused and make me accidentally type into it so I made this simple edit that remembers the focused window for each desktop

the focus restore should silently fail and not cause issues if the window is somehow closed while it's hidden
```

virgo.c

@@ -41,6 +41,7 @@ typedef struct {
 	Windows desktops[NUM_DESKTOPS];
 	Trayicon trayicon;
 	HWND pinned[NUM_PINNED];
+	HWND focused[NUM_DESKTOPS];
 } Virgo;
 
 static void *stb__sbgrowf(void *arr, unsigned increment, unsigned itemsize)
@@ -339,8 +340,10 @@ static void virgo_go_to_desk(Virgo *v, unsigned desk)
 		return;
 	}
 	virgo_update(v);
+	v->focused[v->current] = GetForegroundWindow();
 	windows_hide(&v->desktops[v->current]);
 	windows_show(&v->desktops[desk]);
+	SetForegroundWindow(v->focused[desk]);
 	v->current = desk;
 	trayicon_set(&v->trayicon, v->current + 1);
 }