fixes https://github.com/spesmilo/electrum/issues/3315
The cause was that tracebacks look different whether stack items have source text available.
When using the pyinstaller windows binary, there is no source text available.
Example when running from source:
```
>>> a
Traceback (most recent call last):
File "...\electrum\gui\qt\console.py", line 256, in exec_command
result = eval(command, self.namespace, self.namespace)
File "<string>", line 1, in <module>
NameError: name 'a' is not defined
```
Example for pyinstaller windows binary:
```
>>> a
Traceback (most recent call last):
File "electrum\gui\qt\console.py", line 256, in exec_command
File "<string>", line 1, in <module>
NameError: name 'a' is not defined
```
When the console was moved between screen boundaries with different scaling
settings, the font was reset. This is because QPlainTextEdit sets the
documents default font back to its own font property when Qt has a font
change event. This patch sets the font property of the editor instead of
the document.
this ports 60d63b3272
related: https://github.com/Electron-Cash/Electron-Cash/issues/1314
sys.ps1 and sys.ps2 define the strings used as primary and secondary
prompts in the Python interpreter.
Also fix a rare bug introduced by myself in 7772af6 (#6607) where
spaces at the end of the current line would not be remembered when
switching server.
Change >> to >>> as in a normal Python console.
Avoid printing an empty string which is why the console looked like:
>>
Network banner
>>
Instead of:
Network banner
>>>
