FindGTK2: Mark GTK_*_INCLUDE_DIR as advanced
FindGTK2: Find freetype quietly
FindGTK2: Search for modules quietly when needed
FindGTK2: Export GTK2_XXX_FOUND to parent scope in _GTK2_FIND_LIBRARY

Also add a debug message to report if the library was found or not
FindGTK2: Populate GTK2_DEFINITIONS before searching for libraries
FindGTK2: Search for glib-object.h instead of gobject/gobject.h

This is the topmost include file for GObject header files
FindGTK2: Create targets for each library

Methods:
 * _GTK2_ADJUST_LIB_VARS
 * _GTK2_ADD_TARGET_DEPENDS_INTERNAL
 * _GTK2_ADD_TARGET_DEPENDS
are strongly inspired by FindQt4.cmake
FindGTK2: Better handling of include directories

Add a method _GTK2_ADD_TARGET_INCLUDE_DIRS to handle it.
FindGTK2: Do not require the GTK_ prefix in all the internal functions

This saves from using string(REGEXP) to create targets
FindGTK2: Small cleanup
FindGTK2: Link freetype libs to targets including freetype includes
FindGTK2: Fix gmodule, glibmm, pangoft2, and pangoxft targets
FindGTK2: Add check to ensure that target exists
FindGTK2: Set INTERFACE_COMPILE_DEFINITIONS target property only if not empty
FindGTK2: Add config directories only if different from include ones
FindGTK2: Do not link libfreetype

As discussed on the mailing list, freetype includes used in GTK2
headers libraries do not require to link the library explicitly (even
though it is already linked by GTK2 libraries.

Also remove _GTK2_ADD_TARGET_LIBRARIES no longer used and use
${FREETYPE_INCLUDE_DIR_ft2build} ${FREETYPE_INCLUDE_DIR_freetype2}
variables instead of ${FREETYPE_INCLUDE_DIRS}
FindGTK2: Add libraries to the GTK2_LIBRARIES variable only when found

Some libraries (e.g. gio) are not necessary, and often not available
with older GTK2 versions, therefore GTK_LIBRARIES should not contain
GTK2_XXX-NOT_FOUND for these libraries.
FindGTK2: Do not add freetype includes if they are not found

On some older system they are not required
FindGTK2: Refactor _GTK2_ADJUST_LIB_VARS into _GTK2_ADD_TARGET

This function avoids creating the targets when the required
dependencies were not found.
Also fix some wrong dependency and some typo.

${FREETYPE_INCLUDE_DIR_ft2build} ${FREETYPE_INCLUDE_DIR_freetype2} are
now required for gtkmm component
FindGTK2: do not skip target creation if optional dependencies are not found

On some older GTK2 version gio, giomm and cairomm are not available