Add named save states widget, disable menu entries for numbered/quick…

[Giragast]

Allows for save states to be given names (beyond appending 0-9) and be created, overwritten, loaded & deleted within a bespoke widget

The naming format is [GAMEID]_[NAME_OF_SAVE_STATE].sstate

These named save states are also listed in the File->Load save slots menu

This PR also disables menu items for save states that don't have an existing file to load, helping to indicate at a glance which are currently in use

[ghost]

👇 Click on the image for a new way to code review

Legend

[codecov]

Codecov Report

Attention: 142 lines in your changes are missing coverage. Please review.

Comparison is base (a072e38) 8.59% compared to head (948bdca) 8.59%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/gui/widgets/named_savestates.cc	0.00%	101 Missing ⚠️
src/gui/gui.cc	0.00%	40 Missing ⚠️
src/gui/widgets/named_savestates.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1466      +/-   ##
==========================================
- Coverage    8.59%    8.59%   -0.01%     
==========================================
  Files         408      410       +2     
  Lines      125609   125742     +133     
==========================================
  Hits        10802    10802              
- Misses     114807   114940     +133     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.