Preserve plugin order when reinstalling/updating

[jurassicplayer]

Please tick as appropriate:

• I have tested this code on a steam deck or on a PC
• My changes generate no new errors/warnings
• This is a bugfix/hotfix
• This is a new feature

If you're wanting to update a translation or add a new one, please use the weblate page: https://weblate.werwolv.net/projects/decky/

Description

This fixes issue: #513

This change preserves the plugin order if the plugin is already installed and assumes that when calling _install on an already installed plugin, either reinstall/update was used and the entry should in theory still be there after install (this assumption can fail if the install fails). This should preserve the order across full reboots and should be tested with full reboots after plugin changes (re-installs and updates, but no harm in checking plugin installs and deletions as well).

[nabel0]

Tested and works fine under SteamOS Stable/Beta/Preview