libgimp,plug-ins: split metadata loading into prepare() and finish()

So the plug-in has the chance to decide whether it wants to trust the
metadata information (e.g. resolution). Also reorder parameters in
gimp_image_metadata_save_finish(). Change all plug-ins accordingly.
plug-ins: reverse the logic for setting GimpMetadataSaveFlags

start with flags = ALL (which now includes all possible current and
future flags), and optionally *remove* individual flags instead of
adding them. This way the plug-ins default to TRUE for future flags.
libgimpbase: fix typos in gimpmetadata.c
plug-ins: trust the PNG's resolution over its metadata's resolution
plug-ins: trust the TIFF's resolution over its metadata's resolution
plug-ins: trust the TIFF's resolution over its metadata's resolution

This also sortof addresses bug #691231, but fixes the issue only
indirectly by not perferring the EXIF resolution.
plug-ins: trust the PSD's resolution over its metadata's resolution
libgimp: add API docs to libgimp/gimpmetadata.c
libgimpbase: add API docs to libgimpbase/gimpmetadata.c
devel-docs: update docs for libgimpbase
devel-docs: update docs for libgimpmath
devel-docs: update docs for libgimpcolor
devel-docs: update docs for libgimpconfig
devel-docs: update docs for libgimpwidgets
libgimp: use gexiv2_metadata_get_tag_interpreted_string() for the comment

So the charset="foo" is stripped away and we get the string as it is
supposed to be presented to the user.
app: save XCF version 6 when the image has metadata
app: should also register a loader for XCF version 6...
Updated Brazilian Portuguese translation
build/win: Fix string comparison in split-build for non-bash shells.
plug-ins: move the metadata plug-in from File to Image