Ensure that wxPOPUP_WINDOW on Mac always sets the window level
to NSPopupWindowLevel, regardless of the wxSTAY_ON_TOP and
wxFRAME_TOOL_WINDOW flags.
As the comment for wxPOPUP_WINDOW in defs.h says,
"set this flag to create a special popup window:
it will be always shown on top of other windows"
So, that should *always* be true, even in relation to other
Windows with wxSTAY_ON_TOP, since NSPopUpMenuWindowLevel
is greater (higher) than NSModalPanelWindowLevel.
It should also *always* be true even if the window has both
wxPOPUP_WINDOW and wxFRAME_TOOL_WINDOW styles, since there's
no reason why we can't have a panel with NSUtilityWindowMask
and NSPopUpMenuWindowLevel.
For wxImage having both mask and alpha channel (it is technically possible), mask cannot be converted to alpha values and in this case resulting wxBitmap will also have both mask and alpha channel.
Due to lack of support for horizontal scrolling in the native control,
scrolling it was implementing by offsetting the entire control window.
However this didn't work correctly when the window was not positioned
at the leftmost border of its parent window, as the part of it that was
scrolled off could still be visible in this case, and this is exactly
what happened when the native header was used in wxGrid: scrolling it
overwrote the corner part of wxGrid.
Fix this by embedding the actual native control inside an outer wxWindow,
to ensure that the scrolled off part is clipped by the parent window.
Note that this commit is best viewed with "git show --color-moved" as
most of the code was just moved from the header into the implementation
file and is not really new.
Override wxHeaderColumn::GetMinWidth() to return the actual minimum
width instead of just returning 0.
Add a unit test verifying that this works as intended.
Drag-resizing the columns didn't work correctly when using the native
header and scrolling it horizontally, as the wrong offset was used in
this case.
Fix the offset in wxGrid code and add a unit test checking that this
works as intended (at least under MSW, as wxUIActionSimulator just
doesn't work reliably enough to test for this under the other platforms,
and, besides, only MSW has the native header control implementation
anyhow).
