Add colorbox on thumnail pages

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Move thumbnail code to dedicated class

This also moves the code from mfile->image_dimension() to the only
place where it was called.

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Rework image manipulation class

This is the first of hopefully more classes using namespaces and proper
classes that can be used as objects rather than CI's singleton
approach. The namespace is mainly used to gain nice autoloading
capabilities and it's not really yet used for separation.

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Support EXIF orientation in colorbox

This also reindents colorbox.css.

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Only use colorbox if it's loaded

Signed-off-by: Florian Pritz <bluewind@xinu.at>