Linux: support copy&paste functionality (XF86Copy, XF86Paste, XF86Cut, XF86Undo, XF86Redo, etc...)

I initially reported this bug/suggestion to Telegram Desktop team: https://github.com/telegramdesktop/tdesktop/issues/5835

But it was found that this is the issue that applies to Qt-based applications including Clementine-Player, Qbittorrent and obviously Telegram Desktop.

My original report to Telegram Desktop team:

I am a keyboard power user and in that I am greatly helped by the DreymaR's Extend. The idea is simple: use the separate Extend key (by default mapped to CapsLock key), which opens up a bunch of functionality without the need to move my fingers too much.

In practice it's even more amazing and useful than one could imagine.

The problem is that the basic functionality of Cut/Copy/Paste/Undo/Redo doesn't work in Telegram. It works in OS, it works in Firefox, it works in LibreOffice and everywhere else, but sadly it doesn't work in Telegram Desktop.

Steps to reproduce

1. Install [Extend.
2. See for yourself that although other buttons work, the Cut/Copy/Paste/Undo functionality doesn't work in Telegram (for example in Firefox/LibreOffice everything works fine).

Expected behaviour

XFree86 has a bunch of XF86 keyboard symbols and the vast majority of them are not relevant to Telegram or other typical programs in daily use. But there is a number of them that are highly relevant. At the very least:

• XF86Copy,
• XF86Paste,
• XF86Cut,
• XF86Undo,
• XF86Redo
• (- XF86Back?)
• (- XF86Forward?)

It would be very nice to fix to support them.

If one has time, one could additionally look at implementing other shortcuts such as:
(- XF86Clear?)
(- XF86Go?)
(- XF86History?)
(- XF86Search?)

Configuration

Operating systems:
Linux Mint 19.1 Cinnamon (4.0.9). - Linux Mint 20.1 Cinnamon (4.8.6)

Version of Telegram Desktop:
1.6.1. - 2.7.3

Used theme:
Classic.

https://github.com/telegramdesktop/tdesktop/issues/5835