Provide Scrollable.setScrollbarsMode to add the possibility to change the scrollbars mode

[wiresketch]

Is your feature request related to a problem? Please describe.
We've encountered a performance issue with GEF/Draw2d and overlay scrollbars on Linux/GTK3. See eclipse#430.

This performance issue can be avoided by disabling overlay scrolling using GTK_OVERLAY_SCROLLING=0 environment variable. However this is not a good solution since it impact all scrollable controls in the application and not only the specific control with have performance issues with. Also in GTK4 this variable is not supported anymore so this solution won't work in the future (see https://docs.gtk.org/gtk4/running.html#environment-variables where GTK_OVERLAY_SCROLLING variable is not listed anymore).

A more appropriate solution would be to add Scrollable.setScrollbarsMode(int) method that would allow to disable overlay scrolling for a specific scrollable control without affecting the rest of the application.

Describe the solution you'd like

Add Scrollable.setScrollbarsMode(int) method that would allow to change the scrolling mode for a specific Scrollable control. On GTK3 and GTK4 this can be easily implemented using the gtk_scrolled_window_set_overlay_scrolling method.

On Windows as far as I can see this method would be noop since overlay scrollbars are not supported there.

I am not familiar with macOS API, but it looks like it would be possible to set scrollerStyle property on NSScrollView.

[iloveeclipse]

@wiresketch : would be nice if you could contribute a patch.

[wiresketch]

I've created a pull request and added the required implementation for GTK. On Windows I've left the method as noop.

On macOS I've also left the method as noop. I am not sure if this method needs such an implementation there. Those with more experience on macOS please chime in.

[iloveeclipse]

Thanks for contributing!