Replace copy-assigning GUI<T>::GetSetting from rP31 with the reference returning GetSetting from rP22693.

Avoids one or two dozen unoptimizable string copies and two CPos copies, mostly in SetupText.

The reference return allows to mark values as const where the previous one prevented that.
This also reveals unused variables where the previous code hid them.

Thus use unused variable "buffer_zone" in the otherwise unused (since rP14476, refs rP18419) CCheckBox::SetupText() from rP2532 and
use the unused variable "scrollbar" in CInput GUIM_MOUSE_PRESS_LEFT from rP1904.

Refs unintentionally copied DrawCall cache due to GetSetting copying in rP1507, fixed in rP1518.

Differential Revision: https://code.wildfiregames.com/D2215