Scale Outline controls by natural width

Previously, the Outline/Shadow spinboxes and the Border Style combobox
shared a fixed sizer proportion of 1. Because the combobox has a much
wider natural size than the spinboxes, the wxBoxSizer forced the
spinboxes to stretch excessively to match the combobox's width.

This changes `add_with_label` to use the control's natural width
(`GetBestSize().GetWidth()`) as its sizer proportion instead of a
fixed value.

By weighting the proportion based on width, the layout achieves two
goals:
1. At the default dialog size, the spinboxes are not forced to expand
   to match the wider combobox, allowing them to retain their compact,
   natural size.
2. When the dialog is resized horizontally, all controls expand
   together, distributing the extra space relative to their original
   widths, rather than the combobox remaining fixed or the spinboxes
   growing disproportionately.

Author: 0tkl

src/dialog_style_editor.cpp

@@ -145,7 +145,7 @@ DialogStyleEditor::DialogStyleEditor(wxWindow *parent, AssStyle *style, agi::Con
 
 	auto add_with_label = [&](wxSizer *sizer, wxString const& label, wxWindow *ctrl) {
 		sizer->Add(new wxStaticText(this, -1, label), wxSizerFlags().Center().Border(wxLEFT | wxRIGHT));
-		sizer->Add(ctrl, wxSizerFlags(1).Left().Expand());
+		sizer->Add(ctrl, wxSizerFlags(ctrl->GetBestSize().GetWidth()).Left().Expand());
 	};
 
 	auto num_text_ctrl = [&](double *value, double min, double max, double step, int precision) -> wxSpinCtrlDouble * {