Can't paste to gdb prompt/terminal

**Describe the bug**
Can't paste to gdb prompt/terminal

**To Reproduce**
1. Run `gdbgui -r --args ....`
2. Open browser on remote host.
3. Everything looks good. I can type in the lower/left terminal box and interact with gdb. I can hover over vars. RHS works.
3. See messages in lower center box: "Copy/Paste available in all terminals with ctrl+shift+c, ctrl+shift+v"
4. Try to paste into lower left gdb terminal with ctrl+shift+v, numerous other *-v, firefox right button menu.
5. No form of paste works.

**Expected behavior**
Paste should transfer data from my clipboard to the gdb prompt.
I want to paste breakpoints ("b asdfasdfasd").

**Please complete the following information:**
* Ubuntu 18.04
* gdbgui 0.14.0.2
* gdb 8.1.1
* firefox on Ubuntu 20.04

```
# pipx list
venvs are in /root/.local/pipx/venvs
apps are exposed on your $PATH at /root/.local/bin
package gdbgui 0.14.0.2, Python 3.6.9
- gdbgui
```

```
## pipx runpip gdbgui freeze
Brotli==1.0.9
click==7.1.2
dnspython==2.1.0
eventlet==0.25.2
Flask==0.12.5
Flask-Compress==1.8.0
Flask-SocketIO==2.9.6
gdbgui==0.14.0.2
gevent==1.5.0
gevent-websocket==0.10.1
greenlet==0.4.16
itsdangerous==1.1.0
Jinja2==2.11.2
MarkupSafe==1.1.1
monotonic==1.5
pkg-resources==0.0.0
pygdbmi==0.10.0.0
Pygments==2.7.4
python-engineio==3.14.2
python-socketio==4.6.1
six==1.15.0
Werkzeug==0.16.1
```

Start by reproducing the issue with `gdbgui -r --args ....` and the listed Firefox setup, then trace the lower-left gdb terminal's paste handling. Done means clipboard text can be pasted into the gdb prompt, including a breakpoint command such as `b asdfasdfasd`, using the documented shortcuts or browser menu.

Written by the indexing model from the issue text.