Change "Key bindings" to "Keyboard shortcuts"

[jeanprbt]

This PR updates the Preferences tab for Key bindings, renaming it to Keyboard shortcuts. It updates the tab title, the page title and the messages when changing keyboard shortcuts. This follows a request from @koppor on the Gitter channel.
This PR updates the file in resources/l10n/JabRef_en.properties to replace all occurrences of key\ bindings with keyboard\ shortcuts.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

The "key binding" should not appear in the language file any more. If it does, you missed some string. (Or adapt the PR description)

Reason: We do not keep unused strings.

[jeanprbt]

Sorry about that, I updated the PR description. All occurrences of "key bindings" are in fact replaced by "keyboard shortcuts".

[Siedlerchr]

Changelog entry please under "changed" section