Differentiates better between the EDA_ITEM IsModified(), referring to
items themselves changing and the EDA_SCREEN IsContentModified(),
referring to whether we have made any unsaved changes.
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
The groundwork has now been laid for per sheet instance data. Initially
this only supports sheet page numbers but could be expanded to include
other per sheet instance information.
ADDED: Support for user defined schematic page numbers.
