Shows iRA, iDEC instead of RA, DEC for markers in 2D while projection are not correctly implemented here

Makes markers textarea readonly

fix bug : when a 2D image was generated from a 3D cube, the 2D image was loaded instead of the 3D one when it was reloaded

this was because in Datablock.createOneSlice we found an instruction :
hdu.header = Header

were Header was the reference to the Header of the 3D file. Then all modifications on hdu.header afterwards  modified the 2D and the 3D headers at the same time.

code was replaced by :

hdu.header = copy.copy(Header)

to work on a copy of the header. Same problem was in createSummedSlice and createSmoothCube