GeneralScrollPane no longer exists in Pharo 12

[JanBliznicenko]

GeneralScrollPane was renamed to GeneralScrollPaneMorph few Pharo versions ago with the original GeneralScrollPane being first deprecated, then finally removed in Pharo12. Magritte uses GeneralScrollPane in 3 places.

The direct usage of this class can be replaced by UIManager default newScrollPaneFor: that seems to work across all versions of Pharo (tested Pharo 3-12). It, however, would not work for other Smalltalk implementations.