Previously, the em <-> pixels conversion was a hardcoded value. Now the GUI scales with the font size that can be selected in the options.
Sadly, all scales were off since the hardcoded value was too low.
All calls that reference an existing Gui window now start with "GuiUpdate" (exception: GuiClose). All other functions are always to be called DURING menu creation and layouting.
The "Margin" property replaces Gui_AddMargin.
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/CustomGuiOpen/GuiOpen/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/CustomGuiClose/GuiClose/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/CustomGuiSetTag/GuiUpdateTag/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/CustomGuiUpdate/GuiUpdate/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/Gui_AddCloseButton/GuiAddCloseButton/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/Gui_UpdateText/GuiUpdateText/g'
find . -type f -name "*.c" -print0 | xargs -0 sed -i 's/Gui_AddSubwindow/GuiAddSubwindow/g'
Before you'd define the X coordinate of the top-left corner of a window with X = [500, -40], now you do Left = "50%-4em". The em values currently do not scale with font-size yet, but it is important to do the formal change as early as possible to not have to do all work twice. Scaling of font and thus menus can be implemented later.
This should reduce the confusion about why the function behaves so differently from Gui_Open: the style objects only provide an interface for those easy GUIs, they are not really part of the GUI system.