Editor: Support floating bottom dock and improve frame spinbox stepping

[Naruto]

Description

• Added support for floating bottom docks (Make Floating) in Godot 4.3+:
  • Previously, calling playback_panel_button->hide() forced the floating dock to close. This has been resolved by keeping the tab button visible, allowing the floating window to remain active while clearing the selected player context instead.
  • Since Godot 4.3's add_control_to_bottom_panel disables floating by default for backwards compatibility, the available_layouts property of the parent dock is now explicitly set to 6 (Horizontal | Floating).
• Improved UX for the current frame SpinBox in the playback panel:
  • Maintains the precision required for smooth playback updates (step = 0.01), but introduces a custom stepping of 1.0 when users click the up/down arrow buttons.
  • This utilizes Godot 4.3+'s new custom_arrow_step property safely via set() to maintain compatibility with older Godot 4.x GDExtension headers.