Update desktop file and app description to match AppStream data

This was updated to "Chat on Matrix" but in other places it was never
switched from "Matrix client" and the like. Now it should be more
consistent.

(cherry picked from commit f31c644b13)

org.kde.neochat.desktop

@@ -87,7 +87,7 @@ GenericName[uk]=Клієнт Matrix
 GenericName[x-test]=xxMatrix Clientxx
 GenericName[zh_CN]=Matrix 客户端
 GenericName[zh_TW]=Matrix 用戶端
-Comment=Client for the Matrix protocol
+Comment=Chat on Matrix
 Comment[ar]=عميل لميفاق ماتركس
 Comment[az]=Matrix protokolu üçün müştəri
 Comment[ca]=Client per al protocol Matrix

src/main.cpp

@@ -140,7 +140,7 @@ int main(int argc, char *argv[])
     KAboutData about(QStringLiteral("neochat"),
                      i18n("NeoChat"),
                      QStringLiteral(NEOCHAT_VERSION_STRING),
-                     i18n("Matrix client"),
+                     i18n("Chat on Matrix"),
                      KAboutLicense::GPL_V3,
                      i18n("© 2018-2020 Black Hat, 2020-2024 KDE Community"));
     about.addAuthor(i18n("Carl Schwan"),