Install several Flatpaks at once

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Feature
Add the capability to install multiple at once.

Situation: New install, want to install several flatpaks from flatpakref files I have stored on a separate drive.

**Describe the solution you'd like**
Select several .flatpakref files in chosen file manager, right click, open in Sideload, sideload shows you info on all apps selected and can tell you the full install size and proceed to install them.

**Existing work**
Not that I've seen.

**Describe alternatives you've considered**
If GUI is too much a CLI option, `sideload flatpak1.flatpakref flatpak2.flatpakref flatpakn.flatpakref`

**Additional context**
Keep up the good work.