Fix namespaces for Value() and Color() functions.

Without "ImGui::" linker fails to find those function definitions:

Undefined symbols for architecture x86_64:
  "ImGui::Value(char const*, float, char const*)", referenced from:
      blah-blah in blah.o
  "ImGui::Value(char const*, unsigned int)", referenced from:
      blah-blah in blah.o
ld: symbol(s) not found for architecture x86_64