Enable middle mouse button paste for Linux

[t-bltg]

Enable middle mouse button pasting selections from other opened views into an opened terminal.

It has been bothering me for a while, on Linux we often use middle paste to quickly copy/paste some code from other views. This PR seems to fix the issue reported here: https://stackoverflow.com/questions/65246456/middle-click-is-not-pasting-the-text-in-terminus-sublime-text-3

This PR allows:

• double click to select a single word in another view & middle paste into a terminal;
• select some code in another view by mouse click + select + mouse release & middle paste into a terminal.
• select some text in an application other than ST & middle paste into a terminal.

Linked issue: https://github.com/randy3k/Terminus/issues/158

Result: 1. typing OK, 2: middle paste OK:

[t-bltg]

@randy3k I've minimized the changes, and the current solution works on my side, even with a lot of output in the terminal. Furthermore, is does not affect Win / Mac users.

Does it seem acceptable to merge this ?

[randy3k]

Sorry for the late reply. LGTM. It is sad that I don't even have a mouse to test it.

[t-bltg]

Thanks :+1:

[randy3k]

I have just ordered a mouse from Amazon. I really want to test it first before making a release.