Vcs: Pimpl plugins

Essentially rename all *Plugin into *PluginPrivate, and pull out
the actual IPlugin related pieces into new *Plugin classes.

Shift the construction of the PluginPrivate to initialize(),
following the general pattern.

I tried to keep the patch as mechanical as possible, giving
room to some obvious but less mechanical cleanup needs,
that are intentionally left out of this here.

Change-Id: Iac662bf73338f9f7669064ed67b960246875c23c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/settingspage.cpp

@@ -124,7 +124,7 @@ void SettingsPageWidget::updateNoteField()
 // -------- SettingsPage
 
 SettingsPage::SettingsPage(Core::IVersionControl *control, QObject *parent) :
-    VcsClientOptionsPage(control, GitPlugin::client(), parent)
+    VcsClientOptionsPage(control, GitPluginPrivate::client(), parent)
 {
     setId(VcsBase::Constants::VCS_ID_GIT);
     setDisplayName(SettingsPageWidget::tr("Git"));