WIP: Update Gtk API calls in key editor

[cydanil]

Description

This PR fixes the launching the key editor: first by updating GTK API calls from GtkComboBoxEntry to GtkComboBoxText, and second

How Has This Been Tested?

This was tested by changing keys, items, and units of various ingredients, both one at a time and several at once.
Searching for either of keys, items, and units was also tested.

Screenshots (if appropriate):

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.

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 26ecf2e into 1d43a22 - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging 0575567 into 1d43a22 - view on LGTM.com

fixed alerts:

• 1 for Unused import