file-marks: scroll to last position in file when opening.
gtk: add more util helpers for gtk

These helpers add some convenience functions for things we've been doing
all over the place manually.

It also adds a workaround for the inability to scroll to the right offset
until textlayouts have been processed enough to know where our offset is.
snippets: add guard snippet
snippets: remove some debugging code
editor: save file mark when closing a tab
editor: add function to restore file mark in an editor tab.
editor: load file mark after opening a file

This now uses workaround code that works around faulty Y positions in
gtktextview.