R.app macOS GUI improvements and fixes

[villegar]

Originally proposed as a project for the R Project Sprint 2023; however, it was decided to keep working on this after the sprint. The original issue can be seen here: r-devel/r-project-sprint-2023#39

Originally posted by s-u July 6, 2023
The R for Mac GUI is the most comprehensive platform-specific R GUI and has been pioneering a lot of features now present in other GUIs and commercial products, but in recent years it has not been updated to take advantage of any of the new macOS features. This idea is an umbrella for any work of the R for Mac GUI which includes:

• fixing any reported bugs related to the GUI; see on Bugzilla
• improve modern features or visuals (e.g., better integration with the dark mode, modernise the setting panel); a reference to Apple's design guidance
• improve/add translations: new translations for the GUI added (including Spanish -completed-, Arabic, Hindi and Turkish) via Weblate
• propose interesting features that can be implemented reasonably quickly (e.g. usability/ergonomic features)

[barracuda156]

@villegar @s-u If someone helps me with XIBs (they are not recognized by Xcode 3.2.6, the last ones compatible are from R-app 1.70), I can probably fix the current master for Darwin ppc in a way that will be mergeable.

Here is 1.79 (XIBs are borrowed from 1.70):