Don't close menu(s) on certain operations

In several cases we don't want menu(s) to close when an action on menu was
done. For example on drag-n-drop or menu item deletion (where confirmation
dialog is shown).

In such situations `s_bPreventClosing` was set to true (and then back to
false when closing was allowed again).
Though original code honored this variable only in certain situations and
typically (at least on Win10) menus were hidden/closed despite of it.

This patch changes the behavior and menus(s) are not closed when
`s_bPreventClosing` is set to true.
Basically now menu(s) stay visible until there is an action that changes
active window.

Following functionality was also removed because it is not needed now:

* CMenuContainer::HideTemp
* COwnerWindow::OnClear
  WM_CLEAR was sent to the window only by already removed `HideTemp`

SetupHelper: Try to terminate ClassicShell Start Menu (to make upgrad…

…e a bit easier)

During upgrade from ClassicShell we'll try to terminate
ClassicStartMenu.exe as well. So that installer won't complain about files
being in use.

Add information about my clean script

Add information about my clean script

Remove AppVeyor auto build test triggering file

Remove AppVeyor auto build test triggering text file

Update README.md