Office Add-In Manifest overrides for dark mode

As per https://stackoverflow.com/questions/65441392/does-outlook-web-app-support-dark-mode-changes-for-manifest-xml.

It is difficult and limiting for add-ins to provide a single set of icons that have good contrast in both light and dark colour schemes. Getting the contrast right is important for meeting accessibility guidelines.

This could be solved by enabling the add-in manifest to specify overrides for the Image element based on colour scheme.