common: install xsce-extra repo

Fixes #107
common: change task name
Merge pull request #109 from migonzalvar/install-xsce-extra-repo

common: install xsce-extra repo
Modify release notes from candidate to final.
Update INSTALL.rst

Clarified ansible installation
Merge pull request #111 from tim-moody/release

[REVIEW] [MERGE] Modify release notes from candidate to final.
Merge pull request #112 from annabham/install-notes-update

Update INSTALL.rst
Merge branch 'master' of https://github.com/georgejhunt/xsce

upstream and origin got out of sync
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