tree: Fix various typos and outdated terminology

This was mostly machine generated with the following command:
```
codespell \
    --builtin clear,rare,usage \
    --skip './po/*' --skip './.git/*' --skip './NEWS*' \
    --write-changes .
```
using the latest git version of `codespell` as per [these
instructions](https://github.com/codespell-project/codespell#user-content-updating).

Then I manually checked each change using `git add -p`, made a few
manual fixups and dropped a load of incorrect changes.

There are still some outdated or loaded terms used in GLib, mostly to do
with git branch terminology. They will need to be changed later as part
of a wider migration of git terminology.

If I’ve missed anything, please file an issue!

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gmenumodel.c

@@ -429,7 +429,7 @@ g_menu_model_class_init (GMenuModelClass *class)
    * @removed: the number of items removed
    * @added: the number of items added
    *
-   * Emitted when a change has occured to the menu.
+   * Emitted when a change has occurred to the menu.
    *
    * The only changes that can occur to a menu is that items are removed
    * or added.  Items may not change (except by being removed and added