Upgrade to DocBook 4.5 instead of 4.3
Most of our DocBook XML files use the dtd version 4.3, but there are also some that use 4.5. There are in fact very little differences and I did not see any changes when changing all to 4.5.
Although there is also a version 5, from what I have read, that may need more extensive changes. So, I don't think we should upgrade to that at this time (with our limited resources).
There is one reason why I did not commit this change. Certain version of xmllint produce validation errors after changing all files to 4.5: error : xmlAddEntity: invalid redeclaration of predefined entity
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This seems to be an error in xmllint, but we don't want this to cause validation errors in gimp-help. See e.g. libxml2#308 (closed). Since there is nothing wrong, except different files using a different DocBook version, let's keep it the way it is for now.
Let this issue be a reminder, that we should do this change sometime in the future.