Add a boolean "compact" style property for GimpSpinScale. When
TRUE, the widget uses a narrower layout, and the different upper/
lower-half behavior is gone. Instead, the behavior depends on the
mouse button used: left-click is used for absolute adjustment
(similar to the upper-half behavior), middle-click is used for
relative adjustment (similar to the lower-half behavior), and right
click is used for manual value entry (similar for clicking on the
text area).
Add a new "Compact sliders" toggle to the Interface prefernces, to
control the spin-scale style. Apply the style globally through the
themerc file, and update it when the option changes.
Use the compact style by default, because otherwise no one would
find it. Theming in GTK3 works differently, and spin scales in
master need more work regardless, so this stays in 2.10 for now.
8591b5b33f