Compose key plugin: expand "%L" to full path.

According to [1] "%L" should expand to a full path for the
default (based on system's locale) Compose file.

[1] http://www.x.org/archive/current/doc/man/man5/Compose.5.xhtml

Task-number: QTBUG-35943
Change-Id: Ie803a89742d9c0aa3b2d759bea28ed403dc68c9c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>