WIP: Disable plain text converters to support Unicode with XWindowsClipboard

[pdecat]

This PRs aims to resolve issue #344, where Unicode characters pasted to linux client or server are replaced by question marks.

This is marked as WIP as I'm still investigating why the text/plain converter is selected instead of the Unicode one.

The current change is more like a work-around that disables plain text converters for
XWindowsClipboard. Did not notice side effects yet.

[pdecat]

Note: the Windows Clipboard had its plain text converter disabled years ago: 3ddb7ef#diff-8d79106a107fa1e6578a4bb0cde4db76L46

[AdrianKoshka]

Thanks for looking into this issue and contributing.

[pdecat]

I did not dig any further for the root cause. No issue with this change so far for me.

To avoid breaking stuff for others, maybe this could be made an option defaulting to old behavior.
Or take a little more risk and make the default depend on the user's locale:

• *.UTF-8 => disable plain text converters by default
• anything else => keep them enabled by default

[eizanprime]

I tried this PR and it works very well, I can copy-paste unicode text from windows client to linux server with no problem now (which would get replaced by ?? without this PR)

[p12tic]

The fix is more like a hack, we need to figure out why correct converter is not selected. I suspect the real bug is in that part of the code.

[pdecat]

There is this fix in upstream that may be related https://github.com/symless/synergy-core/pull/6617

[shymega]

Related to #860.

[yan12125]

Based on this PR, I created another PR #1164. I believe that one is a correct fix. Hope it helps :)

[pdecat]

Superseded by #1164