Generate changelogs of Themes

As of today, only plugins changelog is semi-auto generated. This is a
problem when theme is upgraded and not its version number. So now the
themes have their section in the changelog.

Special dedicac^Wnote for Experimental theme: it *includes* things from
common pseudo theme. If something is changed in common, the version
number of Experimental should be changed if we want the new stylsheets
to be built during package generation.

Change-Id: Ie62fa9291c584e0579d14a2f944caeac5ef6188a
Merge commit 'refs/changes/28/1828/2' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Generate changelogs of Themes

Change-Id: I8e175eb667adf1362a58e84e7ec951dbf327c4d4
Tuleap 6.10.99.4

Change-Id: Iaabba577a3f04867aed217a62f9769df32dada01