in GetMsgPanelInfo() (PPI, scale).
m_image name was confusing: it is not a wxImage, but a BITMAP_BASE item,
and m_image name was already in use in BITMAP_BASE, used inside SCH_IMAGE.
Bitmap properties dialog: show PPI value and add Title to dialog.
Converts dialog_image_editor to panel_image_editor. Embeds this panel in
a new properties dialog for the schematic and PCB editors that allows
editing position, layer, locked status, etc. like other items.