Make FileFormat::newInstance const.

These shouldn't be non-const, and making them const allows
the FileFormatManager to return const pointers to its internal
FileFormat objects that can't be used for reading/writing, but
can be identified and cloned.

Change-Id: If46564fef9b0a31deb4a2f8a9689154a00420ff4
Make built-in format identifiers easily identifiable.

Add "Avogadro: " prefix to the built-in format identifiers
so that they can be easily distinguished from other formats
by users.

Change-Id: Ia3860918d331ee5b9dc19de1a25c3b5928b2d3b2
Add methods to get all FileFormats for a given extension.

Change-Id: Icd982e81b8294473028c5d53cec85835748201a6
Merge topic 'file-io-selection' into master

fb9b6ac1 Add methods to get all FileFormats for a given extension.
9d028ca0 Make built-in format identifiers easily identifiable.
10675841 Make FileFormat::newInstance const.