Desktop: add multi-sync input mode

[SashankBhamidi]

Adds multi-sync input mode (Altiris-style) — keyboard and mouse input from the active tab gets replicated to all other open VNC connections.

What's in this:

• Menu toggle under Machine → Multi-Sync Input (Ctrl-M)
• Mouse replication with coordinate scaling for different framebuffer sizes
• Key replication (down/up/char) + modifier release on focus loss
• [Sync] tab prefix on non-active tabs as visual feedback
• Auto-disable when <2 connections remain
• i18n entries for de, es, sv

Menu toggle screenshot

Tab sync prefix screenshot

Closes #216

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #216: multi-sync input mode (similar to altiris)

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[SashankBhamidi]

Hi @bk138, feel free to retarget this PR to a feature branch if you'd like to keep it off master until v0.11.0 is out.

I don't remember clearly, but before merging or closing the issue, you’ll need to approve and accept my PR on issuehunt first: https://oss.issuehunt.io/r/bk138/multivnc/issues/216

[SashankBhamidi]

Hi @bk138, just checking that what you’re working on won’t cause any conflicts with this branch. I won’t be able to revisit it for about three weeks starting Friday, so wanted to confirm. Let me know.

[bk138]

@SashankBhamidi thanks for the heads-up, I'll take care of any conflicts that might arise!

[SashankBhamidi]

Perfect, happy hacking!