Fixes to bugs from devs beta testing initiative

• 1. warn user more reliably when overwriting a workspace - https://bugs.kde.org/show_bug.cgi?id=444980
• 2. fix resetting seexpr script when renaming
• 3. disable editing of the name of the script when overwriting to make it more clear what's going on (and changing the name here didn't work anyway)
• 4. Fix crash on loading tmp.kts copy. https://bugs.kde.org/show_bug.cgi?id=444950

Test Plan

• 1. Create a new workspace and see that the button immediately changed to "Overwrite"
• 2. Select a script preset; make visible changes (for example change colors); rename the preset; see that the changes are not lost any more.
• 3. Select a script resource, change it visibly, wait a moment, click "Overwrite" button
• 4. Try to load a task set called tmp.kts copy. If you don't see it in your file dialog, just write down the filename and press Enter regardless of the "Save" button being greyed out (this method works on Qt dialogs).

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.