KDE Portal Toggle.

### Feature Description

Add Toggle option for switching xdg portals from GTK to KDE (QT) portals.

### Use Case

It will allow user to change portals to be changed from settings instead of diving into the config files.

Reason for requesting: if using niri everytime niri is updated it revers all the portals to GTK instead QT.

it's specially problem on git version as it is updated frequently.

### Compositor(s)

- [x] All compositors
- [x] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

Similar to include line is add to niri config this will add default portal lines to the xdg config.

### Alternatives/Existing Solutions

_No response_

Start by tracing the settings UI and the existing niri config/include handling, then determine how default xdg portal lines are represented in the xdg config. Done means users can switch between GTK and KDE (Qt) portals from settings and the selected defaults persist without manual config edits.

Written by the indexing model from the issue text.