Add compatibility for Safari Web Extension

[sebastianlivoni]

This pull request adds compatibility for use in a Safari Web Extension. The idea is to embed this keepassxc-browser extension within the KeePassXC app on macOS. The pull request for this is here.

Non-supported features in Safari Web Extension:

• The ability to change keyboard shortcuts dynamically (showing unsupported info dialog for this)
• Autofill HTTP Auth dialogs (showing unsupported info dialog for this)
• Show notifications for errors when user interaction is required. (hiding this option)

Screenshots or videos

Testing strategy

• Conducted progressively as development continued.

Type of change

• ✅ New feature (change that adds functionality)

[varjolintu]

Few things to fix in the files:

• Leave the empty line change at the end
• Use single quotes ' instead of "
• Use translations in the HTML files instead of plain English text

[phoerious]

Also, can we use our dark-theme logo for dark toolbar backgrounds? Simply inverting the light-theme logo doesn't work.