Change animation button from "play" to "pause" when animation is running, and add animation speed button.

[ameraner]

Currently, when an animation is running, no change in the animation buttons can be seen. This can be confusing in certain situations, e.g. when the animation speed is very low or too fast (screen update not keeping up), and the user doesn't know if the animation is running or not.

This could be improved by switching to a classic pause button || when the animation is running, or keeping the "play" button pressed until it is clicked on again.

In addition, the animation speed slider can only be changed when right-clicking the "play" button, which is not intuitive/clear to a new user. A new button or always-visible slider should be added.

Ported from EUM Gitlab issue 173.

[ameraner]

Relevant piece of code:

sift/uwsift/__main__.py

Lines 292 to 295 in ac7d431

	def toggle_animation(self, action: QtWidgets.QAction = None, *args):
	"""Toggle animation on/off."""
	new_state = self.scene_manager.animation_controller.toggle_animation()
	self.ui.animPlayPause.setChecked(new_state)