Fix workspaces and saving brush presets

• fixed adding a workspace in Window -> Workspaces -> New Workspace: https://bugs.kde.org/show_bug.cgi?id=444112
• fixed managing workspaces in the workspace/window layout chooser (the button on the toolbar)
• fixed creating new brush presets
• fixed sessions saving in a wrong place when closing Krita: https://bugs.kde.org/show_bug.cgi?id=443652
• fixed updating a brush tip or other similar resources from Krita as an image: https://bugs.kde.org/show_bug.cgi?id=443875

Test Plan

1. Create a workspace in Window -> Workspaces -> New Workspace (previously never asked for a name and always failed)
2. Create a workspace in the workspace chooser, make sure it lands in the workspaces area
3. Create a new brush preset, make sure it doesn't ask you weird questions, as if it was still considering the preset to be the preset you started your editing from (the one you based your new brush preset on).

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.