Ensure Mac's titlebar padding always leaves space for the window controls

[p-jackson]

Related to #541 and #596

This PR fixes the same issue as #596 except this time for Mac. Namely, the padding in the app's top bar needs to be aware of both the RTL mode of the language and the RTL mode of the underlying browser window.

Proposed Changes

• Refactor isWindowFrameRtl logic into utility module
• Create a <MacTitlebar> component that does the equivalent job of <WindowsTitlebar>
  • i.e. adjust padding to accommodate windows controls
  • Linux continues to the same topbar padding logic as Mac. No change here.
• Ensure the padding we leave for Mac window controls is based on the browser window's language, not the language used by @wordpress/i18n etc.
• Use a fixed pixel width for the window controls, rather than some number of rem
• Did some fix ups in the site's sidebar to use logical properties, which can automatically deal with RTL switches.
  • I did a side-by-side comparison with trunk to confirm none of the padding sidebar padding has changed.

RTL language in a RTL window

LTR language in a RTL window

LTR language in a LTR window

RTL language in a LTR window

Testing Instructions

Here's a quick way to change the browser window's language without having to reboot your OS into a different language: add app.commandLine.appendSwitch( 'lang', 'ar' ); at the very top of index.ts, right after the import statements.

• Test a LTR language in a RTL window
• Test a LTR language in a LTR window
• Test a RTL language in a RTL window
• Test a RTL language in a LTR window

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[sejas]

Thanks for making the changes for Mac. I tested it the four scenarios LTR and RTL language in the two "frames". Nice tip using app.commandLine.appendSwitch( 'lang', 'ar' );

Screenshots:

[p-jackson]

app.commandLine.appendSwitch( 'lang', 'ar' );

Yeah it'd be nice to boot the app with the correct switch given the user's setting I think.

[wojtekn]

Thanks for the fix @p-jackson !