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/vcsbase/vcsbasesubmiteditor.cpp

@@ -509,7 +509,7 @@ static QString withUnusedMnemonic(QString string, const QList<QPushButton *> &ot
 }
 
 VcsBaseSubmitEditor::PromptSubmitResult
-        VcsBaseSubmitEditor::promptSubmit(VcsBasePlugin *plugin,
+        VcsBaseSubmitEditor::promptSubmit(VcsBasePluginPrivate *plugin,
                                           bool *promptSetting,
                                           bool forcePrompt,
                                           bool canCommitOnFailure)