Clipboard: Don't use "text/plain"

On X11, not all programs offer this target, e.g. some terminals like
XTerm. The target "UTF8_STRING" has worked fine for many years and it is
what dmenu uses.

On Wayland, wl-paste supports the special type "text", which attempts to
autodetect a suitable type (in case there is no exact match for
"text/plain").

lib/menu.c

@@ -1155,11 +1155,11 @@ bm_menu_run_with_key(struct bm_menu *menu, enum bm_key key, uint32_t unicode)
             {
                 char *paster[2] = {NULL, NULL};
                 if (key == BM_KEY_PASTE_PRIMARY) {
-                    paster[0] = "wl-paste -t text/plain -p";
-                    paster[1] = "xclip -t text/plain -out";
+                    paster[0] = "wl-paste -t text -p";
+                    paster[1] = "xclip -t UTF8_STRING -out";
                 } else {
-                    paster[0] = "wl-paste -t text/plain";
-                    paster[1] = "xclip -t text/plain -out -selection clipboard";
+                    paster[0] = "wl-paste -t text";
+                    paster[1] = "xclip -t UTF8_STRING -out -selection clipboard";
                 }
 
                 for (size_t paster_i = 0; paster_i < sizeof paster / sizeof paster[0]; paster_i++) {