Hide quick format bar when selection is cleared

I noticed a bit of an annoying behavior with the quick format bar. Whenever I make a typo when writing a message I usually go back with a ctrl+shift+arrow selection to correct it, and that of course triggers the bar, but then there wasn't any obvious way to make it go away (other than pressing backspace or delete, [ChatBar.qml#L339](https://invent.kde.org/network/neochat/-/blob/master/src/chatbar/ChatBar.qml?ref_type=heads#L339)), so it'd always be stuck visible until I clicked somewhere with my mouse, which I found somewhat annoying.

This intends to fix that, by hiding it whenever the selection is cleared, which seems like a reasonable and expected behavior to me. I would also be okay with making it dismissable with escape, if this current suggestion has some unintended side-effect that I've missed.

I also think that this should in practice "solve" this bug https://bugs.kde.org/show_bug.cgi?id=511590 but I didn't link it directly as their suggested solution is smarter placement of the bar.

src/chatbar/ChatBar.qml

@@ -286,6 +286,8 @@ QQC2.Control {
                             quickFormatBar.selectionStart = selectionStart;
                             quickFormatBar.selectionEnd = selectionEnd;
                             quickFormatBar.open();
+                        } else if (quickFormatBar.visible) {
+                            quickFormatBar.close();
                         }
                     }