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.

It seems like shared_from_this may not be enabled when a unique_ptr is
assigned to a shared_ptr (although it *should* be enabled when
constructing a new shared_ptr from a unique_ptr). Functions that return
objects that may need to use shared_from_this now return shared_ptr,
just to be safe.

Additionally, in some cases, shared_from_this was being called from
Thread::run after the last reference to the shared object had been
released. We now call shared_from_this during 'open', which will always
run on the message thread while at least once reference to the shared
object is alive.

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 change makes heavyweight views listen to window events from higher
in the window hierarchy. If a move or resize event from higher in the
hierarchy is encountered, blocking modal components will be dismissed.

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 change will make plugin views (and other heavyweight views) listen
to their containing window, and dismiss any blocking modal components,
such as PopupMenus when the window is moved or resized.

This should fix issues where opening a combobox then moving the plugin
window could leave the combobox's popupmenu 'stranded'.

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

This reverts commit a6217ae9fb.