MdeModulePkg UiApp: change code for easy customization.

MdeModulePkg UiApp: change code for easy customization.

Change file arrangement to let user easy customize the front page UI.
After this change:
1. UiCustomizeFrontPage function in FrontPageCustomizedUi.c use to
let user customize front page menus. UiFrontPageCallbackHandler
function use to handle the callback for the added menus.
2. FrontPageCustomizedUiSupport.c file used to add support functions
used by FrontPageCustomizedUi.c file.
3. After this change, only above two files need to be updated if front page
menus needed to be changed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>