Many sections either do not have a support of Size/Content or support just a
one of them, e.g only Content.
Section is the base class for sections. This patch adds Content and Size members
to it and removes similar members from derived classes. This allows to cleanup and
generalize the code and adds a support of these keys for all sections (SHT_MIPS_ABIFLAGS
is a only exception, it requires an unrelated specific changes to be done additionally).
I had to update/add many tests to test the new functionality properly.
Maybe am early return instead to be more consistent with other sections?