patch 8.1.1399: popup windows not adjusted when switching tabs

Problem:    Popup windows not adjusted when switching tabs.
Solution:   Save and restore first_tab_popupwin. Fix closing a tabpage.
diff --git a/src/testdir/dumps/Test_popupwin_02.dump b/src/testdir/dumps/Test_popupwin_02.dump
new file mode 100644
index 0000000..c2aa7a2
--- /dev/null
+++ b/src/testdir/dumps/Test_popupwin_02.dump
@@ -0,0 +1,10 @@
+| +8#0000001#e0e0e08|+| |[|N|o| |N|a|m|e|]| | +2#0000000#ffffff0|[|N|o| |N|a|m|e|]| | +1&&@49|X+8#0000001#e0e0e08
+> +0#0000000#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @6|o+0#0000001#ffd7ff255|t|h|e|r| |t|a|b| @10| +0#4040ff13#ffffff0@46
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|:+0#0000000&|c|a|l@1| |p|o|p|u|p|_|c|r|e|a|t|e|(|'|o|t|h|e|r| |t|a|b|'|,| |{|'|l|i|n|e|'|:| |4|,| |'|c|o|l|'|:| |9|}|)| @2|0|,|0|-|1| @8|A|l@1|