This new factoring is a bit more typesafe, as it avoids casting
internals to void*.

It also allows cursors to scale appropriately on high resolution
displays on Windows.

Previously, BadMatch errors were seen when there were several modal
windows, each with a native titlebar. Moving a window would attempt to
restack the windows, which was not possible because the JUCE windows
were not siblings. We actually need to restack the top level windows,
i.e. the windows containing the server-side decorations.

This resolves a regression which was introduced in
74ca3b44c4.

Prior to that commit, drag and drop on Linux/X11 worked as expected.
DragAndDropContainer::performExternalDragDropOfFiles allowed dropping
files onto other applications.

After the faulty commit, this function may cause the window manager to
enter a bad state in which the drag operation never finishes, making it
impossible to switch between windows or close windows.

I think the issue happens because the DnD source may receive (spurious?)
XdndLeave messages during the drag, which cause the peer's entry in the
dragAndDropStateMap to be removed. Before the faulty commit, each peer
had its own drag and drop state object, which was not destroyed in this
way.

This change will keep the peer's drag state object alive, even when an
XdndLeave message is received. The entry will still be removed when the
peer's window is destroyed.

As specified in https://specifications.freedesktop.org/clipboards-spec/clipboards-latest.txt, CLIPBOARD should be used for explicit cut/copy/paste operations over PRIMARY

Previously, things like PopupMenus which were created in their own
windows were not being scaled correctly on HiDPI displays on Linux.

This patch forces the display scale to 1.0 in plugins, meaning that
the transform applied to the main plugin window is the sole source
of truth for component scaling in plugins.

This patch should resolve an issue where the popupmenu for a combobox
could become 'stranded' if the plugin window was moved while the box was
open.

This behaviour was already present on windows, so this patch updates
Linux and macOS window-handling to match.

Previously we were choosing a Visual and Colormap on initialisation of the X11 Display which could lead to bugs when opening a window with different visual and colour requirements. This commit makes 16, 24 and 32-bit visuals available and chooses the correct one when opening a window.