[Pdfedit-support] Make CPage::addSystemType1Font() return font ID of added font

[Gwurk <gw...@fi...>]

Currently, if we add a font with CPage::addSystemType1Font(), it is
hard to tell what the font ID of the added font will be. The function
tries "Fonticek", "Fonticekk", "Fonticekkk", etc., until it finds one
that is unused, but we don't know which one it picked.

I propose changing the function to return the font ID of the added
font. Also, instead of using "Fonticek", I propose using "F1", "F2",
etc., for the font ID (the first one that's available), which is how
many other PDF implementations generate font IDs.

I've attached a patch implements this change. Thanks.