Keep RichTextLabel visible character properties in sync

The RichTextLabel class is inconsistent in how it updates the
visible_characters and percent_visible properties when either is changed.

To keep both properties consistent, update percent_visible when setting the
visible_characters property.
For both properties, when setting one, notify change for the other.

Docs updated for member set_visible_characters on RichTextLabel class.

(cherry picked from commit 463e2002abc0ecd7439fbf00966cef83842babce)

doc/classes/RichTextLabel.xml

@@ -330,6 +330,7 @@
 		</member>
 		<member name="visible_characters" type="int" setter="set_visible_characters" getter="get_visible_characters" default="-1">
 			The restricted number of characters to display in the label. If [code]-1[/code], all characters will be displayed.
+			[b]Note:[/b] Setting this property updates [member percent_visible] based on current [method get_total_character_count].
 		</member>
 	</members>
 	<signals>

scene/gui/rich_text_label.cpp

@@ -2688,6 +2688,7 @@ void RichTextLabel::set_percent_visible(float p_percent) {
 		visible_characters = get_total_character_count() * p_percent;
 		percent_visible = p_percent;
 	}
+	_change_notify("visible_characters");
 	update();
 }
 
@@ -2870,6 +2871,15 @@ void RichTextLabel::_bind_methods() {
 
 void RichTextLabel::set_visible_characters(int p_visible) {
 	visible_characters = p_visible;
+	if (p_visible == -1) {
+		percent_visible = 1;
+	} else {
+		int total_char_count = get_total_character_count();
+		if (total_char_count > 0) {
+			percent_visible = (float)p_visible / (float)total_char_count;
+		}
+	}
+	_change_notify("percent_visible");
 	update();
 }