* GUI internals refactoring * unify string conversion calls part1
This commit adds an indirection for function module GUI_IS_AVAILABLE via interface ZIF_ABAPGIT_GUI_FUNCTIONS and factory ZCL_ABAPGIT_UI_FACTORY. For tests the implementation can be replaced with ZCL_ABAPGIT_UI_INJECTOR=>SET_GUI_FUNCTIONS
- create popups with factory ZCL_ABAPGIT_UI_FACTORY - inject test doubles with injector class ZCL_ABAPGIT_UI_INJECTOR