tree: Fix various typos and outdated terminology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There are still some outdated or loaded terms used in GLib, mostly to do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>

gobject/gsignal.c

@@ -1545,7 +1545,7 @@ g_signal_new (const gchar	 *signal_name,
  * of a class offset for the signal's class handler. This function
  * doesn't need a function pointer exposed in the class structure of
  * an object definition, instead the function pointer is passed
- * directly and can be overriden by derived classes with
+ * directly and can be overridden by derived classes with
  * g_signal_override_class_closure() or
  * g_signal_override_class_handler()and chained to with
  * g_signal_chain_from_overridden() or
@@ -2623,7 +2623,7 @@ g_signal_connect_data (gpointer       instance,
  *
  * Blocks a handler of an instance so it will not be called during any
  * signal emissions unless it is unblocked again. Thus "blocking" a
- * signal handler means to temporarily deactive it, a signal handler
+ * signal handler means to temporarily deactivate it, a signal handler
  * has to be unblocked exactly the same amount of times it has been
  * blocked before to become active again.
  *