Desktop: Set external apps as favorites

[djsime1]

What's new

• Revised favorite app struct from ID to location (internal/external) and name/file.
• Selecting internal favorite app remains the same.
• Selecting "Applications" opens file browser to select a .fap file.
• Also fixes the issue of favorites changing if app ID changed.

Verification

• Select "Applications" as a favorite app to verify you're prompted with the file browser.
• Once a external application is selected, launch it from desktop with LEFT button.
• If the application is removed, FAP loader shows that the file is missing.

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[djsime1]

Tentative demo video:

1664732180_qFlipper.mp4

Won't be updating until #1795 and #1812 get merged.

[djsime1]

Original video is now outdated, file browser entries load app name and icon. The loading spinner is also displayed while the app is loading. Everything should be ready to go!

[djsime1]

1665160995_qFlipper.mp4