Update sigil to 1.2.1

Summary:
Changelog:

• New Features
  • Provide a tweak setting to adjust drag distance
  • Added YesToAll and NoToAll options for replacing images via Add Existing…
  • Highdpi and uifont setting information is now available to Sigil’s plugins
  • Add a Checkpoint feature to Sigil for fast versioned saves to git repositories
  • add ability to easily create a Checkpoint
    • add ability to see the changes between the current epub and any earlier Checkpoint
    • add ability to revert the entire epub back to an earlier Checkpoint
    • added ability to manage and delete Checkpoint repositories.

• Bug Fixes
  • Fix cosmetic file name issue when aborts file rename edits in BookBrowser
  • Improve default drag-distance for BookBrowser
  • Fix index generation bug related to non-escaped text used in regular expressions
  • Fix bug that caused lost class attributes when marking words for an index
  • Guide to Landmark Semantic mappings have been extended to better cover “other.*” guide items
  • Modified the codebase to use Ordered hashes/dictionaries throughout to prevent unneeded reordering in tag attribute order

Resolve T9125

Signed-off-by: Algent Albrahimi <algent@protonmail.com>

Test Plan:

• From an EPUB file, edit the text.
• Add and remove images.
• Add a video file and preview it.
• Check spelling.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Maniphest Tasks: T9125

Differential Revision: https://dev.getsol.us/D9201