small UX update for optionsdialog

• add enableApplyButton() and disableApplyButton() to optionsdialog.{h/cpp}
• they are used to ensure the Ok button does not get disabled, when Apply needs to be disabled (standard UX should allow Ok always to dismiss the dialog and only disable it, when we have a faulty proxy IP)
• disable Apply after initially loading the settings, as nothing new needs to be saved
• remove orphan settings from optionsdialog.ui that are default anyway