Qt: add quicksave/quickload hotkeys (slot 1)

[JoJoKorok]

Summary

This adds two new configurable hotkey actions to the Qt frontend:

• Quicksave state (slot 1)
• Quickload state (slot 1)

The hotkeys call the existing savestate code paths and do not change savestate behavior for menu actions.

Motivation

Quicksave/quickload hotkeys are a common quality-of-life feature in emulators and are particularly useful for:

• controller-based play
• quick testing during gameplay
• speedrun practice and general usability

Currently, savestates require using SHIFT + F(NUM) to assign it the corresponding save state number and F(NUM) to load said save state. This provides a simple, deterministic alternative without introducing new savestate logic.

Implementation details

• Refactors savestate save/load handling into shared helper functions
• Adds new hotkey identifiers and exposes them in the Input & Hotkeys configuration UI
• Wires hotkey presses through the emulation thread to the main window
• Uses a fixed slot (slot 1) for quicksave/quickload in this initial version

Notes

• No changes to core emulation logic
• No behavior changes to existing save/load menu actions
• Feature is limited to the Qt frontend

Testing

• Built and tested on Windows (MSYS2)
• Verified that:
  • menu-based save/load behavior is unchanged
  • quicksave hotkey saves to slot 1
  • quickload hotkey restores from slot 1
  • appropriate OSD messages are displayed