Header files are parsed incorrectly as main files

[yvesdm3000]

Reported by I_inc at http://forums.codeblocks.org/index.php/topic,20623.msg144772.html#msg144772

Header files are often parsed incorrectly as main files in their own translation unit. Every file in a project has a property (Properties... -> Build -> Compile file) specifying whether it is a separate translation unit. This property could be a basis for the decision. Besides it would be nice to be able to quickly switch every such header between existing translation units, in which the header file is included, through a quickly accessible menu, because it's not uncommon to have (very) different parsing of the same header depending on the translation unit.