[WIP/RFC] Multi-selection

[techydude0713]

Edit: I didn't implement this for the CLI yet, will do so soon.

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Based off of PR #102
A rework of multiselect, using more Qt functionality (I had no idea the feature was a thing, so thanks @dhtdht020!)
Changes from the previous PR:

• No INI support, this will done at a later point
• Change the list style into a checkable list
• Simplified functionality (Although it seems like many edits, it's mostly indenting)
• The font issue was fixed (caused by me in the previous PR).
  • When using Qt Designer/Creator, it automatically changed the font property to my system's default font.
  • This causes Windows to use it's default font, instead of Segoe UI.

The functions originally created were reused, but with slight modification, especially for the use of item checkboxes.

I did see that Qt has a multiselect feature, but I wasn't happy with how it looked.
(Personally, I liked the colored boxes that represents the download status).

[techydude0713]

For the CLI, should apps be individual parameters (oscdl.py get app1 app2 ...), use a simple text file (oscdl.py get apps.txt), or both?

[dhtdht020]

For the CLI, should apps be individual parameters (oscdl.py get app1 app2 ...), use a simple text file (oscdl.py get apps.txt), or both?

individual parameters sounds proper.