Middle-button-click doesn't paste in Linux

**Describe the bug**
In Linux, pasting the most recently-selected text, by clicking the middle button, doesn't actually paste anything.

**To Reproduce**
Select text in one window; middle-click in gdbgui window. Nothing happens.

**Expected behavior**
Middle button should paste from secondary clipboard.

**Please complete the following information:**
* OS: Fedora 33
* gdbgui version (`gdbgui -v`): 0.14.0.2
* gdb version (`gdb -v`): N/A
* browser [e.g. chrome, safari]: Chrome
* python packages (`pip freeze`): N/A