Implement support for renaming dynamic launchers

[mwleeds]

Quoting from here:

• When Flatpak renames an app, it looks for launchers in ~/.local/share/xdg-desktop-portal/applications/ and updates both their name and Exec=/TryExec= lines. It also fixes the sym links in ~/.local/share/applications, so there's no time span when the logged in user's launchers are invalid.

(we would also need to rename the icon in ~/.local/share/xdg-desktop-portal/icons/)

This is something we will want to support for the new dynamic launcher portal, but it is an edge case that probably won't come up much, so it doesn't seem critical to make sure it's available everywhere the portal is.