Fix clipped printer selection in Create Filament Preset dialog
Ensure the scrolled panel in the Create Filament Preset dialog expands vertically by changing `m_main_sizer->Add(..., proportion=0)` to `proportion=1`. This exposes the printer selection checkbox, which is required to create a custom filament.
Fixes#6719
Co-authored-by: SoftFever <softfeverever@gmail.com>