Extract IPageList interface in a interface only unit

[MHumm]

as it is the same in both other units and with this one of the duplicate GUIID issues from Mantis 6658 can be resolved.
http://issuetracker.delphi-jedi.org/view.php?id=6558

[obones]

This is definitely not enough because you did not change the package definitions inside the package\xml folder.
Without those changes, JvPageListInterface.pas will be included twice, once in JvPageComps.bpl and once in JvCustom.bpl
This would give hints at compile time that JvPageListInterface.pas is implicitly included in both packages, but even worse, it would give an error when starting the IDE because no two packages can include a file with the same name.
To fix this, JvPageListInterface.pas must be included in a package that both JvCustom and JvPageComps require. There is already JvStdCtrls in that case, so JvPageListInterface.pas should be added to the JvStdCtrls-R.xml file.
Then all packages should be regenerated and the changed files also added to this pull request.

[MHumm]

I have to put that on hold for now, as I get lots of problems compiling the D26 package. Most likely due to a regular GetIt Jvcl installation being on the same VM. Need to get a few things sorted out first before I will take that one up again.