[FEATURE] Add support for mouse moved event

When writing editors with Cursive, it would be really desirable to have a mouse `Moved` event to get the current mouse position. Currently it is only possible to get the mouse position when a button is pressed or scrolled, so there is no way to know which column/row the mouse is hovering at at any time. It would be really useful if, on backends that support it, cursive reported such event (on backends that do not support it, such event would never be reported).