[INTERNAL] Switch FileList from IPath inputs to String inputs

Internally, FileList used Strings for storing information about paths
anyway. The dependency from (Eclipse) IPath could have been replaced by
(Saros) IPath dependencies, but all IPath operations could simply be
replaced by plain String operations, so this is done here.

Furthermore, this patch moves some "builder" logic from FileList to
FileListFactory (i.e. the "addMembers()" method), which eases the
separation of IDE-dependent and IDE-independent logic.

Change-Id: I2e88849619c41a86211f8473a396d3c64d35f4db
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1562
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>