Adds get_selection_line_offset to RichTextLabel

This new method allow you to get the line offset of the current
selection (returns -1 if nothing is selected.)

This is useful if you want to pop up a control or menu above the
currently selected text. Previously there was no accurate way to get
this information.

The logic is moved from the implementation of `scroll_to_selection`
verbatim, and that method has been adjusted to avoid repetition.

doc/classes/RichTextLabel.xml

@@ -190,6 +190,12 @@
 				Returns the current selection first character index if a selection is active, [code]-1[/code] otherwise. Does not include BBCodes.
 			</description>
 		</method>
+		<method name="get_selection_line_offset" qualifiers="const">
+			<return type="float" />
+			<description>
+				Returns the current selection vertical line offset if a selection is active, [code]-1.0[/code] otherwise.
+			</description>
+		</method>
 		<method name="get_selection_to" qualifiers="const">
 			<return type="int" />
 			<description>