Fix warnings in compilation of wrt-plugins-common

Fix warnings in compilation of wrt-plugins-common

[Issue#] N/A
[Problem] Compilation warnings appearing, part 2.
[Cause] Incorrect code.

[Solution] Make some changes. Most of the changes are related to warning
"type qualifiers ignored on function return type". It is generated by
returning a constant pointer from function. I removed unnecessary consts
related with warnings.

[Verification] Build wrt-plugins-common and look at logs of the building.
There should be only two warnings:
 - "Warning: mmap_min_addr is != 0. If programs fail at mmap this could
	be the reason"
 - "warning: 'WrtDB::PropertyDAOReadOnly::WidgetPreferenceList
	WrtDB::PropertyDAOReadOnly::GetPropertyList(WrtDB::DbWidgetHandle)'
	is deprecated (declared at
	/usr/include/dpl-efl/dpl/wrt-dao-ro/property_dao_read_only.h:79)" at
	wrt-plugins-common-0.3.72/src/modules/tizen/WidgetInterfaceDAO/WidgetInterfaceDAO.cpp:112:71

Change-Id: I262c6e5197e84ae83bffef8f23d1ea52295e3adb