Add Support for Custom Component Styles

[Putyk]

Hello, first of all, thank you for your contribution! This plugin is incredibly convenient and practical in actual use.

While using Godot, I often create custom UI components and would like to add custom styles for these components to the theme. Below is a temporary code snippet I implemented:

Add Custom Component

func _add_custom_theme_style(theme: Theme) -> void:
	pass

func add_custom_theme_item(
		theme: Theme,
		name: StringName,                # Component name, e.g., "Timepoint"
		theme_type: Theme.DataType,      # 0-6
		item_dir: Dictionary             # Dictionary of options to add, e.g., {"normal": stylebox}
	) -> void:
	# Set theme items based on type
	match theme_type:
		0:
			for state in item_dir.keys():
				if item_dir[state] is Color:
					theme.set_color(state, name, item_dir[state])
				else:
					push_error("Invalid color value for state '%s'" % state)
		1:
			for state in item_dir.keys():
				if item_dir[state] is int:
					theme.set_constant(state, name, item_dir[state])
				else:
					push_error("Invalid constant value for state '%s'" % state)
		2:
			for state in item_dir.keys():
				if item_dir[state] is Font:
					theme.set_font(state, name, item_dir[state])
				else:
					push_error("Invalid font value for state '%s'" % state)
		3:
			for state in item_dir.keys():
				if item_dir[state] is int:
					theme.set_font_size(state, name, item_dir[state])
				else:
					push_error("Invalid font size value for state '%s'" % state)
		4:
			for state in item_dir.keys():
				if item_dir[state] is Texture2D:
					theme.set_icon(state, name, item_dir[state])
				else:
					push_error("Invalid icon value for state '%s'" % state)
		5:
			for state in item_dir.keys():
				if item_dir[state] is StyleBox:
					theme.set_stylebox(state, name, item_dir[state])
					print(1)
				else:
					push_error("Invalid stylebox value for state '%s'" % state)
		_:
			push_error("Invalid theme data type: %d" % theme_type)
	print("Added successfully")

Additionally, integrate _add_custom_theme_style(theme) into the _generate_theme() function:

func _generate_theme(setup_function: Callable):
	_reset()
	_debug("Setting up theme generation.... (%s)" % setup_function)
	setup_function.call()

	if _save_path == null:
		push_error("Save path must be set before generating the theme. (See set_save_path(...))")
		return

	_debug("Generating theme... (%s)" % _theme_generator)
	var theme = Theme.new()
	_add_custom_theme_style(theme)

Usage Example
Here's how this could be used to add custom styles for a "Timepoint" component:

func _add_custom_theme_style(theme: Theme) -> void:
	# Add Timepoint component styles
	add_custom_theme_item(theme, "Timepoint", 5, {
		"normal": StyleBoxFlat.new(),
		"pressed": StyleBoxFlat.new(),
	})

It would be great if similar functionality could be officially supported in the plugin. Thank you again for your excellent work!