Support Xterm modifier keys for dynamic terminfos

Previously, modified function keys in dyanmic terminals were not
supported (like shift/control/alt + arrow keys).

Even though xterm-compatible terminals were dynamically discovered
and their keys added to the terminfo struct, the Modifiers field
was not supported and therefore the modified Xterm keys were not
added to the keycodes map (prepareXtermModifiers was skipped).

This fixes the issue by setting the Modifiers field when we
discover that the terminal is xterm-compatible.

terminfo/dynamic/dynamic.go

@@ -314,6 +314,8 @@ func LoadTerminfo(name string) (*terminfo.Terminfo, string, error) {
 	// but modern XTerm and emulators often have them.  Let's add them,
 	// if the shifted right and left arrows are defined.
 	if t.KeyShfRight == "\x1b[1;2C" && t.KeyShfLeft == "\x1b[1;2D" {
+		t.Modifiers = terminfo.ModifiersXTerm
+
 		t.KeyShfUp = "\x1b[1;2A"
 		t.KeyShfDown = "\x1b[1;2B"
 		t.KeyMetaUp = "\x1b[1;9A"