Implement logic for deciding when to use FreeType

[gasull]

This implements the logic recommended by @EchterAgo.

I don't have a Windows VM to test this out. So please run the code first.

Related PR: #2235

---

This change is 

[EchterAgo]

I'll do a Windows .exe build of this and test it.

[EchterAgo]

The check seems wrong. It will use (S) when using FreeType on Windows. I think this should be correct:

    @staticmethod
    def _get_schnorr_sigil() -> str:
        """Get the right symbol for the platform"""
        should_use_freetype = False
        if sys.platform in {"cygwin", "win32"}:
            config = get_config()
            if config is not None:
                windows_qt_use_freetype = config.get("windows_qt_use_freetype")
                should_use_freetype = bool(windows_qt_use_freetype)
            if not should_use_freetype:
                # On Qt for Windows the 'ⓢ' symbol looks aliased and bad. So we
                # do this for windows.
                return "(S)"
        # On Linux & macOS it looks fine so we go with the more fancy unicode
        return "ⓢ"

[EchterAgo]

Yea, the check is wrong. Using the code above it works correctly.

[gasull]

@EchterAgo , I fixed the code as you indicated. Please test it again.

I wonder if I should git squash all the commits.

[cculianu]

Any chance we can just save this as an attribute in __init__? E.g. call this once: self._schnorr_sigil = self._get_schnorr_sigil()? Just avoids having to do the same work over and over again for a function that likely will not return anything different each time.

[EchterAgo]

https://ec.loping.net/4.2.4-58-g95fe9a4c9/

Windows without FreeType:

Windows with FreeType:

I agree with @cculianu that the sigil should only be determined once on startup. FreeType can not be enabled/disabled without restarting the process.

[gasull]

Any chance we can just save this as an attribute in __init__?

Done.

I'm not sure if it matters where in __init__ I should call _get_schnorr_sigil(). I did it at the end of it.

[EchterAgo]

When opening the TX dialog:

Traceback (most recent call last):
  File "C:\dev\Electron-Cash\electroncash_gui\qt\history_list.py", line 181, in on_doubleclick
    self.parent.show_transaction(tx, label)
  File "C:\dev\Electron-Cash\electroncash_gui\qt\main_window.py", line 1067, in show_transaction
    d = show_transaction(tx, self, tx_desc)
  File "C:\dev\Electron-Cash\electroncash_gui\qt\transaction_dialog.py", line 55, in show_transaction
    d = TxDialog(tx, parent, desc, prompt_if_unsaved)
  File "C:\dev\Electron-Cash\electroncash_gui\qt\transaction_dialog.py", line 143, in __init__
    self.add_io(vbox)
  File "C:\dev\Electron-Cash\electroncash_gui\qt\transaction_dialog.py", line 545, in add_io
    _('{} = Schnorr signed').format(self._schnorr_sigil)
AttributeError: 'TxDialog' object has no attribute '_schnorr_sigil'

[cculianu]

Yeah the attribute needs to be saved earlier in the __init__ function -- namely before anything like self.update() runs. Probably should be done before any other member function is called...

[gasull]

Yeah the attribute needs to be saved earlier in the __init__ function -- namely before anything like self.update() runs. Probably should be done before any other member function is called...

Done.

[gasull]

@cculianu @EchterAgo Is this good to be merged?