Make recent file icons update on saving

Reload the file icon after saving a file.

I have thought of generating the file icon directly from the KisDocument, but I realized that when KisDocument::sigSavingFinished is emitted, the document may already have been modified by the user, so it is not a very reliable way of generating the file icon.

@dkazakov please check the second commit. I added a filePath parameter to the sigSavingFinished so that it gets the correct file path after exporting a file. The KisDocument sender contains the path of the original document, which would not be what the export function have written to.

BUG: 447492

Test Plan

1. Open a .kra file
2. Check the file thumbnail in the recent files menu
3. Modify and save the file
4. Check that the file thumbnail in the recent files menu has changed.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.