Fix and enhance oneclick installing & custom scheme handler

[Mai-Lapyst]

This PR contains:

• A little typo fix when generating the .desktop file
• Enhancement for the .desktop file (proper idention of the resulting file)
• Enhancement of the oneclick installing process in general: now showing a loading spinner so users dont get confused why nothing hapens in the application

[GingerAvalanche]

I assume you've tested this, and it performs as you want it to? There are no issues with the mods getting marked (or not) as 'installable' as they finish downloading?

[Mai-Lapyst]

No, there are none. Despite that, for potential errors I added a commit that sends any catched error to the frontend to display for the user. Because user can potentially have multiple mods downloading & failing at the same time, I simply added a new button to the row when a mods has errors which opens the error modal.

[GingerAvalanche]

Looks good to me. Thanks for your contribution.