Replace deprecated GTK code

[Thanatomanic]

See https://developer.gnome.org/gtk3/stable/DeprecatedObjects.html. There are several instances where RT uses deprecated code. Before we step up to use GTK4 at some point in the future, it would be good to at least have no more legacy GTK3 code.

• [x] GtkStyle — Deprecated object that holds style information for widgets [4 hits]
• [x] GtkHScale — A horizontal slider widget for selecting a value from a range [3 hits]
• [x] GtkHBox — A horizontal container box [100+ hits]
• [x] GtkVBox — A vertical container box [100+ hits]
• [x] GtkHPaned — A container with two panes arranged horizontally [4 hits]
• [x] GtkVPaned — A container with two panes arranged vertically [5 hits]
• [x] GtkTable — Pack widgets in regular patterns [18 hits]
• [x] GtkHSeparator — A horizontal separator [100+ hits]
• [x] GtkVSeparator — A vertical separator [50+ hits]
• [x] GtkHScrollbar — A horizontal scrollbar [1 hit]
• [x] GtkVScrollbar — A vertical scrollbar [1 hit]
• [x] Stock Items — Prebuilt common menu/toolbar items and corresponding icons [4 hits, Gtk::Stock]
• [x] Themeable Stock Images — Manipulating stock icons [2 hits, Gtk::ICON_SIZE_BUTTON] update only needed/possible when moving to gtkmm4

I'll soon create a branch to work on this.

Edit:

• [x] Figure out what is going on here, because the PACK_SHRINK inside the VBox looks really weird https://github.com/Beep6581/RawTherapee/blob/6454737d0ce48e8d1004018bdda347dd0526dd4c/rtgui/preferences.cc#L727

[Thanatomanic]

@heckflosse or @Floessie could you please advise me on how to fix the following: https://github.com/Beep6581/RawTherapee/blob/6454737d0ce48e8d1004018bdda347dd0526dd4c/rtgui/guiutils.h#L375-L383

The HScale class should be replaced by Scale, but it needs a way to ensure GTK_ORIENTATION_HORIZONTAL is used when instantiated. Normally you would do new Gtk::Scale(GKT_ORIENTATION_HORIZONTAL), but how would I approach that here?

[Thanatomanic]

Also, I managed to create this gem:

Who needs a compact interface 👀

[Thanatomanic]

I think I have managed to replace everything so far. But I need to fix this. PR coming soon.