Update Gtk API calls in KeyEditor plugin

[cydanil]

Description

This plugin needs updates to the Gtk API, in places where the method names changed.
There was also an index error that was not handled correctly when it occurred. This is fixed by changing the way the looping is done.

Both these cause troubles when the plugin is activated, as mentioned in #315 .

How Has This Been Tested?

This has been hastily tested at the moment, I will unmark the WIP status once I'm done.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

closes #315

[cydanil]

This is partially correct, the issue still occurs: a newly recipe gets added to the last page, but its title is not updated when on any other page.
Relaunching Gourmet shows the correct title, so it's something to do with on save that breaks silently and the code for updating the view isn't executed.

[cydanil]

The issue this PR aims to close is resolved here, although there is still an issue with the view not correctly being updated on save when the recipe is not on displayed.
The title gets correctly updated if the recipes in the current page.
That is for another PR.