(refs #7358) Mars to Oxygen updates

+Removed org.eclipse.epp.mpc feature from simantics.desktop
and sdk, as the latest version contained unwanted
org.eclipse.mpc.epp.core.win32. Instead, hand-picked the three
plug-ins that are needed for org.eclipse.epp.mpc (ui, help.ui and core).

+Updated target definition to use new mirror-sites and removed
<includeBundles> defintion that bloated the file.

+Added oxygen-specific feature project and set simantics-desktop.product
to use it.

+Updated simantics.target

+Updated rcp feature to no longer depend on
org.eclipse.core.filesystem.java7 (see number 3 here:
https://help.eclipse.org/neon/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Fporting%2F4.6%2Fincompatibilities.html&anchor=filesystem.java7)

+Included oxygen.feature into pom.xml under root->features
+Added missing source files
+Changed project type of org.simantics.root to maven
+Updated mirror-location url to be correct (dropin no longer
has /repository/ at the end)

Change-Id: Id20a3626dbce81cb2a45036d1983d55c151d7e52