SimpleUI Configuration System

[ananyatimalsina]

This PR introduces an underlying configuration system to SimpleUI, allowing
users to define custom tabs, actions, and screens natively via a
config/init.lua file.

Additions & Changes

1. User Configuration Loader (sui_config.lua)

• Implemented getUserConfig() which securely loads config/init.lua using
  pcall and libkoreader-lfs.
• Catches syntax errors or missing files gracefully, displaying an InfoMessage
  in the UI if the configuration file is malformed.
• Automatically injects configured custom_actions into the global
  ALL_ACTIONS registry on boot, resolving relative icon paths to absolute
  plugin paths.
• Allows users to entirely override the bottom bar tabs order.

2. GUI Menu Protections (sui_menu.lua)

• If the user has hardcoded their tab layout in config/init.lua, the "Arrange
  tabs" GUI editor is safely disabled.
• Tapping the menu item now shows an informative popup explaining that tabs are
  managed via configuration, preventing accidental conflicts between GUI
  settings and the config file.

3. Dynamic Action Routing (sui_bottombar.lua)

• Refactored onTabTap and navigate to properly support dynamically
  registered user actions.
• Introduced _isActionOnly helper to correctly identify custom quick actions.
• Action-only tabs that are not is_in_place are correctly routed through
  navigate(). The homescreen and active menus are torn down, but the active
  tab indicator is preserved correctly on the previous tab instead of getting
  stuck on the quick action.
• Action-only tabs that are is_in_place execute gracefully above the current
  view without changing the tab indicator or closing the homescreen.

4. Custom Fullscreen Widget Support (sui_patches.lua)

• Modified UIManager.show monkey-patching to natively support injecting
  SimpleUI top and bottom navigation bars into user-defined custom widgets.
• Iterates over ALL_ACTIONS on boot and registers any user action marked with
  is_fullscreen_view = true into the INJECT_NAMES table.
• Relaxed the strict widget.title_bar requirement during widget injection. As
  long as a custom widget declares covers_fullscreen = true and its
  widget.name matches the action's ID, SimpleUI will successfully wrap it.

5. Configuration Examples (config/init.example.lua)

• Shipped an example configuration file documenting the schema.
• Includes custom_opds: A working example of a Quick Action tab that natively
  launches KOReader's OPDS catalog (dispatcher:execute).
• Includes my_custom_screen: A working example of how to implement and route a
  completely custom Lua widget wrapped in SimpleUI's navigation context.