Widgets: No scrolling with scrollable ancestor

New ScrollProtected class decorator for widgets.

Suppress value changes by scrolling if a widget:

• has a scrollable ancestor (new)
• is not focused
• and the Shift key is not pressed (new)

Logic implemented for these widgets:

• Inkscape::UI::Widget::SpinButton
• Inkscape::UI::Widget::MarkerComboBox
• Inkscape::UI::Widget::DashSelector
• InkScale (#1587 (closed))

Applied to these dialogs:

• Align and Distribute dialog (combo boxes)
• Export dialog (spin buttons)
• Object Properties dialog (image rendering combo box)
• Spellcheck dialog (language combo box)
• Fill and Stroke (color channel spin boxes)
• Text and Font (font size combo box)

Also makes Inkscape::UI::Widget::SpinButton defocusing more generic by focusing the parent scroll window if no defocus widget was set (improves #314).

Fixes #31 (closed) Fixes #1587 (closed)