When possible use app icon from hicolor theme in *.destop files

Currently icon paths are harcoded in .desktop files so they can't be
replaced by themed icons without editing those files by hands.

Now at first it will try get icon from global theme and if it doesn't
exists than harcoded one.
make sure dir exists before calling ln
Merge pull request #372 from vparitskiy/master

Use application icon from hicolor theme in .desktop files

Signed-off-by: David Aguilar <davvid@gmail.com>
packaging: rename git.svg to git-cola.svg

Fixup the DESTDIR handling in the Makefile and tidy things up.

Closes #359
Suggested-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: add v.paritskiy to the credits

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge branch 'icon-fixups'

* icon-fixups:
  doc: add v.paritskiy to the credits
  packaging: rename git.svg to git-cola.svg

Closes #359
Signed-off-by: David Aguilar <davvid@gmail.com>