patch 8.2.2345: no focus events in a terminal

Problem:    No focus events in a terminal.
Solution:   Add the t_fd and t_fe termcap entries and implement detecting
            focus events. (Hayaki Saito, Magnus Groß, closes #7673,
            closes #609, closes #5526)
diff --git a/src/term.h b/src/term.h
index bff901e..a3d8299 100644
--- a/src/term.h
+++ b/src/term.h
@@ -109,10 +109,12 @@
     KS_CST,	// save window title
     KS_CRT,	// restore window title
     KS_SSI,	// save icon text
-    KS_SRI	// restore icon text
+    KS_SRI,	// restore icon text
+    KS_FD,	// disable focus event tracking
+    KS_FE	// enable focus event tracking
 };
 
-#define KS_LAST	    KS_SRI
+#define KS_LAST	    KS_FE
 
 /*
  * the terminal capabilities are stored in this array
@@ -212,6 +214,8 @@
 #define T_CRT	(TERM_STR(KS_CRT))	// restore window title
 #define T_SSI	(TERM_STR(KS_SSI))	// save icon text
 #define T_SRI	(TERM_STR(KS_SRI))	// restore icon text
+#define T_FD	(TERM_STR(KS_FD))	// disable focus event tracking
+#define T_FE	(TERM_STR(KS_FE))	// enable focus event tracking
 
 typedef enum {
     TMODE_COOK,	    // terminal mode for external cmds and Ex mode