Mouse support for slider

[jwlodek]

Is your feature request related to a problem? Please describe.
Currently, the slider widget does not support mouse inputs. Left mouse clicks should be handled to put the slider in the clicked position.

Describe the solution you'd like
Add implementation of _handle_mouse_press function to the slider widget class in controls/slider.py

Additional context
Implementation should call the Widget superclass _handle_mouse_press function at the end of the function. This way, users could use add_mouse_command to perform an event with the new position of the slider. For example, increase the volume on a volume slider bar to the new volume setting.