Move instruction comments to a separate sourceview

[sonnyp]

Git adds instructions in the form of a block comment at the end of the file, for example

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch master
# Your branch is up-to-date with 'origin/master'.
#
# Changes to be committed:
#	modified:   README.md
#

right now we make this readonly and exclude it from select all.

Since it's not deletable anyway, let's move it to its own readonly gtksourceview to remove readonly/select-all code and simplify

We can also do like https://github.com/small-tech/comet/ and move it a a GtkText