display: headed: Add plugin-util.c for plugin utility

display: headed: Add plugin-util.c for plugin utility

int launch_system_app()
 : Remove dependency to the src/shared/apps.h.
   This is copy of function at the src/shared/apps.h so that it makes
   plugin be able to separated from the deviced core.

int display_plugin_config_touch_wakeup()
 : Remove dependency to the extern g_display_plugin at display-plugin.h
   As the actual data resides in the plugin, it is able to access it
   without referring the core display-plugin.h. For now, only the one
   member, touch_wakeup, is being accessed. Therefore, the function
   dedicated to that member is added.

Change-Id: I6e25d4dd108ffe97da4502a5597822c16743ad15
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>