Clean up the chat dialog code a bit, set the keyboard focus on the chat message field

Summary: Remove unnecessary repetition of code and focusing of "Configure..." button by default that leads to unexpected opening of the chat configuration dialog or closing the chat dialog when you press Enter in the message field.

Test Plan: Open the "Game -> Network Chat..." dialog, add some message and press Enter. Chat does not work as before (I cannot figure up why) but at least it does not open the configuration dialog and does not close either.

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: aacid, kde-games-devel

Differential Revision: https://phabricator.kde.org/D25981