GtkTreeView has an "interactive search" feature built-in, which basically kicks
in with the Ctrl-F shortcut by default. Let's hijack this feature and trigger
our own new search popover which is much more powerful (multi-item selection,
ability to use regexp or glob search by enabling these in Preferences, etc.).
This was an idea by Aryeom who wanted to be able to hit Ctrl-F to search for
layers.
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