build: Make "pretty CSS" part of gwtdraft profile

With this patch, developer no longer needs to mess with extra
defines like this, in order to have "pretty CSS" available:

  DEV_EXTRA_BUILD_FLAGS="... -Dgwt.cssResourceStyle=pretty"

All that's needed now is to enable gwtdraft profile:

  DEV_BUILD_GWT_DRAFT=1

Updated README.developer to describe effects of activating
DEV_BUILD_GWT_DRAFT option.

Change-Id: I143d204d2f77e1e4350787c8ba46f61d4700f361
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>