Introducing QComposeInputContext

When switching from Xlib to xcb platform plugin it was agreed that
XIM is deprecated. Users should be using QT_IM_MODULE to load input context
plugin for a more advance input method framework support.

The proposed solution is to parse the compose file directly from Qt. This
approach removes the overhead of communication protocols used in Xlib and/or IBUS.

TableGenerator class follows [1].

The compose file is searched for in the following order:

1) If the environment variable $XCOMPOSEFILE is set,
its value is used as the name of the Compose file.

2) If the user’s home directory has a file named
.XCompose, it is used as the Compose file.

3) The system provided compose file is used by mapping the
locale to a compose file from the list in
/usr/share/X11/locale/compose.dir.

Regarding step 3 - TableGenerator searches in hard-coded locations for
system-provided compose files. Here I have introcuded a new environment
variable QTCOMPOSE which can be used to prepend an extra location
to be searched.

[1] http://www.x.org/archive/X11R7.7/doc/man/man5/Compose.5.xhtml



Task-number: QTBUG-28183
Change-Id: I76dcfd454f3acc23db98192a3673c1ab2af4425f
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>