PopupMenu stays on top of other windows on focus loss of Neovim-Gtk

[theHamsta]

Describe the bug
At the moment Neovim-Gtk's popover doesn't hide on focus loss of the main window. This causes the popover to be rendered above other windows when I switch to a different application.

A possible fix is to rely on the autohide = true behavior of GTK (https://github.com/Lyude/neovim-gtk/pull/99/files) which would be at the cost of possibly deviating from the state of terminal Neovim for set mouse=. EDIT: "autohide" grabs keyboard input.

One could also handle the focus loss explicitly and bring the Popover back when focus is regained.

Technical information (please complete the following information):

• OS: Ubuntu 23.10 (dev version)
• Neovim version: 0.10-dev
• Neovim-Gtk build version: 6f279b4