Add UI for custom screenlock command

[stefonarch]

Rebased version of https://github.com/lxqt/lxqt-session/pull/322

Does not compile yet:

cpp:91:68: error: ‘useCustomLockCommand’ was not declared in this scope; did you mean ‘useCustomLockCommandKey’?
   91 |     ui->useCustomLockCommandCheckBox->setChecked(m_settings->value(useCustomLockCommand, false).toBool());
      |                                                                    ^~~~~~~~~~~~~~~~~~~~

Needs https://github.com/lxqt/liblxqt/pull/346 Fixes https://github.com/lxqt/lxqt/issues/1664