Rework tray icon style

[Phylu]

This PR takes care of multiple issues related to the tray icon style:

• [x] Making the icon color on new messages configurable to address https://github.com/Foundry376/Mailspring/pull/2433#issuecomment-1658683615
• [x] Reworking the Windows icons to match the full color icons on Mac and Linux adressing https://community.getmailspring.com/t/a-more-readable-windows-tray-icon-new-mail-badge/1119

[foundry376-bot]

This pull request has been mentioned on Mailspring Community. There might be relevant details there:

https://community.getmailspring.com/t/system-tray-icon-notification/7318/2

[foundry376-bot]

This pull request has been mentioned on Mailspring Community. There might be relevant details there:

https://community.getmailspring.com/t/dark-gray-icon-on-windows-dark-mode/7337/1