Implementation of the feature and it's ... usage could improve user experience by 'lightening up' pages by collapsing rarely used regions.

Attached archive contains original sources (as of 1.0.0a2) as well as modified ones.
Modifications mainly concern adding 'collapsed by default feature'.
But also contain some other minor fixes that improve some pages layout.
See more detailed description of applied changes here: http://forums.mantisbt.org/viewtopic.php?p=949#949

(Note that some changes mentioned there (e.g. using collapse_api in bug_file_upload_inc, etc.) are not yet applied)