[FEATURE] Arrow buttons next to sliders

[jesse-wei]

Should look like this

To reproduce, in NeuroRuler/GUI/main.py main(), comment in and comment out some lines.

app.setStyle("Fusion")
MAIN_WINDOW: MainWindow = MainWindow()
# with open(constants.THEME_DIR / settings.THEME_NAME / "stylesheet.qss", "r") as f:
    # MAIN_WINDOW.setStyleSheet(f.read())

This approach prevents the GUI from loading the BSS stylesheet.

There's some work on this in the buttons-on-sliders branch that preserves our own stylesheet, but it's incomplete.