appchooser: bring private interface to feature parity with kopenwithdialog

Part of the code lives in plasma-integration: plasma-integration!109 (merged)

This enables:

• file opening
• turns the search field into a combobox featuring the run history
• manual file path typing so long as the path doesn't conflict with an app entry
• terminal launch options

Test plan

Apply plasma-integration!109 (merged) to plasma-integration and compile it.

Apply this MR here and compile it.

Restart the desktop portal as follows:

killall xdg-desktop-portal
/usr/libexec/xdg-desktop-portal &
systemctl restart --user plasma-xdg-desktop-portal-kde.service

Open Dolphin

Right-click on any file > Open With > Other Application…