Add external TTK themes capabilities

[deajan]

Abstract: Tkinter looks quite old.
There are some really nice looking themes (like SunnyValley Themes), see https://github.com/rdbende/Sun-Valley-ttk-theme or https://github.com/rdbende/Azure-ttk-theme

Themes are installed via:

python -m pip install sv_ttk

While they can be loaded manually after a window.finalise() call, it becomes a real burden to manually theme all FreeSimpleGUI widgets one by one.

Here's a patch that allows to specify alternative paths for ttk themes.
This allows loading any ttk theme you want directly into FreeSimpleGUI and avoid errors like Tkinter error: "can't find package ttk::theme::sv_ttk". Is this a headless server ? when trying to manually load said themes.

Abstract code to show what is possible:

import os 
import FreeSimpleGUI as sg
import sv_ttk # noqa # we don't need to access this, only import to retrieve module path

# Load custom sv_ttk light theme
sg.ADDITIONAL_TTK_STYLING_PATHS = path = os.path.join(os.path.dirname(sv_ttk.__file__), "sv.tcl")
sg.DEFAULT_TTK_THEME = f"sun-valley-light"

layout = [
    [sg.Text("Sun Valley Light Theme Test")],
    [sg.InputCombo(("Option 1", "Option 2", "Option 3"), default_value="Option 1")],
    [sg.Button("OK"), sg.Button("Cancel")]
]

window = sg.Window("Sun Valley Light Theme Example", layout)
while True:
    event, values = window.read()
    if event in (sg.WIN_CLOSED, "Cancel"):
        break
window.close()

Before	After

Of course, this is just a demonstration of a combobox, but almost all widgets gain from these themes.

[deajan]

Also fixes PySimpleGUI/PySimpleGUI#4984