[UI] Adds Saros/I Toolbar and MainPanelView

And renames SarosTreeView to SessionAndContactsTreeView

Change-Id: I6a3ab758794ad070d94174b04157ec2b5aeab93d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1982
Tested-by: Jenkins CI
Reviewed-by: Holger Schmeisky <google@schmeisky.com>