Add sameContentLocale option to filter content relationship suggestions

[wottpal]

Closes #851.

Changes

• Fixes issue where content relationship suggestions (combobox options) where incorrectly filtered by fmContentType (cause: fuse threshold)
• Minor display improvement: Show slug as subtitle in combobox option
• Add option sameContentLocale (defaults to true*) that, if set, filters content relationship suggestions by the active locale of the currently edited file.

*Happy to discuss the default here as it changes the current behavior. I think the scenarios where you actually add relations to files with different locales are very uncommon.

Screenshot

[estruyf]

@wottpal I believe it makes sense to change the default behavior, as, like you mentioned, it might be uncommon to select other locales. Checking out the changes.

[estruyf]

@wottpal great job 👌 - added it to the latest beta version.