Add selection getter methods to LineEdit

[Chaosus]

Adds has_selection, get_selection_from and get_selection_to methods to LineEdit class. They are named like in TextEdit where similar methods are implemented.

Closes godotengine/godot-proposals#61

[akien-mga]

Adds has_selection, get_selection_from_line and get_selection_to_line methods to LineEdit class. They are named like in TextEdit where similar methods are implemented.

LineEdit is a single line, no? So I don't understand what from_line and to_line would be aside from 0?

[Chaosus]

Renamed to get_selection_from_column / get_selection_to_column.

[akien-mga]

Thanks!