Appearance page makes application unresponsive when Breeze cursor theme is installed

[edfloreshz]

The application becomes unresponsive for a few seconds when accessing the Appearance page for the first time after opening the app when breeze-cursor-theme is installed.

Steps to reproduce

1. Install breeze-cursor-theme.
2. Open cosmic-settings
3. Access Desktop > Appearance

This may not be exclusive to Breeze, haven't tested other themes.

[leb-kuchen]

cosmic-settings/cosmic-settings/src/pages/desktop/appearance/icon_themes.rs

Lines 223 to 227 in d01e0d8

	if let Ok(handles) =
	tokio::task::spawn_blocking(|| preview_handles(theme, valid_dirs)).await
	{
	icon_themes.insert(IconTheme { id, name }, handles);
	}

This seems to scale terribly with the amount of themes installed. But I still wonder why it performs so terribly,cause it is supposed to be non-blocking.
Also it runs every time and not just when the toolkit dialog is opened.