Fix Username Changing

Fix username changing. Fix problem with QML and database constrains during username changing.

debug: Finished in thread '0x7f3b637fe640' (src/worker/task_download_account_info.cpp:64, run())
debug: ----------------------------------------------- (src/worker/task_download_account_info.cpp:65, run())
debug: <FUNC> changeAccountUserName() 'src/wrap_accounts.cpp' (src/wrap_accounts.cpp:557, changeAccountUserName())
error: Cannot execute SQL query: UNIQUE constraint failed: account_info.key Der Datensatz konnte nicht abgeholt werden (src/sqlite/account_db.cpp:654, changeUserName())
debug: <FUNC> saveToSettings() 'src/settings.cpp' (src/settings.cpp:54, saveToSettings())
emergency: Object 0x5609d3fc97d0 destroyed while one of its QML signal handlers is in progress.
Most likely the object was deleted synchronously (use QObject::deleteLater() instead), or the application is running a nested event loop.