Fix TabContainer Editor theming and remove Debugger style hacks

[lodetrick]

See: #106164 (comment), #104676 (comment)

This PR fixes a bug in the TabContainer theming, where make_flat_stylebox(p_config.dark_color_1, 0, 0, 0, 0, p_config.corner_radius * EDSCALE) was called to create the stylebox for the TabBar background, but make_flat_stylebox multiplies all the input values by EDSCALE, so EDSCALE was incorrectly applied twice. With the change applied, the tabbar_background stylebox completely covers the bottom panel, meaning that there is no need for the negative extended margin that the Debugger's hack used previously, just the zero margins.

This allows for the hacks used to create the Debugger panel's margins (the BottomPanelDebuggerOverride stylebox) to be removed completely.

[KoBeWi]

This affects all bottom panel nodes that extend MarginContainer. Aside from the debugger, there is also TileSet editor and Shader editor.

vs

[lodetrick]

Thanks for pointing that out, I just made it so theres only one BottomPanel stylebox with normal margins, and the Debugger panel now affects the top margin. This fully removes the extra cases in BottomPanel and the extra StyleBox.

[KoBeWi]

Seems fine.
Once we have EditorDock class, this could be probably automatic.

[lodetrick]

While working on another PR I realized that I missed a reference to the deleted stylebox in editor_node.cpp, so I just pushed to remove that. I checked and there are no more references.