Delay graphics initialization

Delay initialization of graphics subsystem by moving gApplication->InitializeGraphics from static initialisation (when loading library) to a static method called when instanciating gClient. This fixes the Jira issue #ROOT-6106
The underlying issue is a recursive call to JIT, for example when the loading a library is triggered by a symbol resolution lookup, and the static initialization of the library itself try to use the interpreter (e.g. via the plugin manager)
root.exe: interpreter/llvm/src/lib/ExecutionEngine/JIT/JIT.cpp:467: void llvm::JIT::runJITOnFunctionUnlocked(llvm::Function*, const llvm::MutexGuard&): Assertion `!isAlreadyCodeGenerating && "Error: Recursive compilation detected!"' failed.