Mark null project initial screen as zoom-initialized

The variable `m_Initialized` in `BASE_SCREEN` is used by
`SCH_EDIT_FRAME` to mark whether a screen had its zoom level initialized
by the "zoom to fit screen" action. When this variable is `false`, the
function `SCH_EDIT_FRAME::DisplayCurrentSheet()` performs "zoom to fit
screen", modifying the zoom level. This function is indirectly called in
the undo routines, so if `m_Initialized` is not set to `true`, a zoom
change will occur when the user undoes an operation, a behavior that is
undesired.

`m_Initialized` was not initialized to `true` for the null schematic
(the schematic that is loaded if no project is loaded), causing the
aforementioned undesired behavior.

To prevent this, I've changed the `SCH_EDIT_FRAME` constructor to set
`m_Initialized` to `true`, since it zooms to fit screen already. I've
moved `m_Initialized` from `BASE_SCREEN` to `SCH_SCREEN`, as it is used
only in Eeschema, and renamed it to `m_zoomInitialized`, a name I
believe that better describes what this variable does.

I've also introduced the function `SCH_EDIT_FRAME::initScreenZoom()` to
group the "zoom to fit screen" action with setting `m_Initialized` to
`true`, as they often should occur together.

I'd also like to say that I'm not confident whether
`SCH_EDIT_FRAME::DisplayCurrentSheet()` should perform the zoom level
initialization at this point, but I have decided to not change this
behavior for now, as the commit history suggests it's several years old.

Fixes https://gitlab.com/kicad/code/kicad/issues/7343

common/base_screen.cpp

@@ -35,7 +35,6 @@ wxString BASE_SCREEN::m_PageLayoutDescrFileName;   // the name of the page layou
 BASE_SCREEN::BASE_SCREEN( EDA_ITEM* aParent, KICAD_T aType ) :
     EDA_ITEM( aParent, aType )
 {
-    m_Initialized       = false;
     m_virtualPageNumber = 1;
     m_pageCount         = 1;         // Hierarchy: Root: ScreenNumber = 1
     m_Center            = true;