exiv2extractor: add support for bmp, gif, webp, tga

Summary:
Gives only minimal info for bmp, gif & tga, but knowing the size
can be useful already.

exiv2 supports bmp, gif & tga since 0.17, so for all versions since
min required version 0.21. Webp is supported since 0.26.

Reviewers: Baloo, Dolphin, astippich

Reviewed By: astippich

Subscribers: aacid, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18434