Visual indicators for tabs

During my initial setup of the `Encryption` plugin I noticed there was no visual indication in the main window of which tabs were encrypted and which weren't. It would've been helpful for troubleshooting to see this.

Another benefit might be knowing that a tab will prompt for credentials before actually clicking it (when tabs aren't hidden).

There could be an indication in the main window title (when tabs are hidden) and on the tab title itself (when tabs aren't hidden). This solution gave me the idea that it would be nice to see which tab I'm in in the main window title (when tabs are hidden).

Perhaps not the most pressing feature but curious about general feedback.

Environment info:
- OS: Windows 10
- CopyQ: 8.0.0
- GnuPG: 2.4.5