GUI: rm dependence of Model on View

A dependence of Model on View has recently slipped in: ProjectDocument.h includes MainWindow.h.

added GUI label

MainWindow::ViewId should be moved to somewhere.

Recently ViewId enum was duplicated in GUI/View/Tool/ID.h. As far as it was used only in MainWindow at that time, I left it only there (commit 37a7d8a4).

But GUI/View/Tool/ID.h still belongs to View directory so it should not be used here.

assigned to @m.svechnikov

mentioned in merge request !1342 (merged)

mentioned in commit 4fcc6cdb

closed with merge request !1342 (merged)