Bug fixes, improved sliders, better exceptions and focus change event

• Added silent exceptions mode.
• Added custom exceptions for different cases.
• Renamed DropDown.cs file.
• Fixed panel scrollbar update - so it won't be called during interaction, to prevent bugs while scrolling and size changes.
• Fixed dropdown problems when inside scrolled panel.
• Better sliders and scrollbars behavior when changing max value.
• Fixed scrollbars to move one step when clicking on edge.
• Fixed sliders and scrollbars default steps count value.
• Fixed sliders and scrollbars when inside a panel with scrollbar.
• Added focus changed event and callback.
• Added default empty value property to text inputs.