Add ability to use middle mouse button for pasting text from primary buffer

[Andrew15-5]

After the answer in #3017 and a little bit of testing I found out that when mouse_mode is true both zellij and neovim can interact with the mouse (it works just like regular GUI windows where you can focus/unfocus a window and at the same time click some buttons inside it). But with mouse_mode false nothing will work. Hold on, the MMB (middle mouse button) does! With mouse_mode true it didn't work (only in Neovim).

Without zellij I can do both: use MMB for pasting text and interact with Neovim, but with zellij I now have to decide which feature I value the most. I don't like that. Can this be fixed? This will make zellij even more transparent (in terms of terminal interaction) than now.

P.S. At least in Linux and at least in X11 you can use "primary selection" or you can use "clipboard" (from man xclip). There is also "secondary selection" but I think no one has either implemented it to work, or it's just not popular/useful.