Adjusted position for showing text selection context menu properly.

Adjusted position for showing text selection context menu properly.

[Title] Adjusted position for showing text selection context menu properly.
[Issue#] N_SE-38751
[Problem] Select All context menu show wrong position in NPRuntime.
[Cause] Context menu is showned at center position of selection range.
        If center position of selection range is in invisible editor box area(outside of the editor box),
        context menu is showned at wrong position and wrong context menu is showned.
[Solution] Adjusted position for shwoing context menu to center position of visible selection range in the editor box.

Change-Id: I6bc8e9c994050e0fbead15525b3d5c9c63cfe9df