Convert the bootstrap module to a proper launcher plugin

[Janrupf]

Description
The bootstrap module is a hack which should not exist in first place. The functionality of it should go into a launcher plugin, as it currently makes the launcher not standalone anymore by preloading jars.

Expected behavior
The bootstrap module is a launcher plugin instead of the hacky bootstrap.

Proposal

1. The launcher starts up and begins its plugin loading cycle
2. Bootstrap is the only launcher plugin which is initially on the path
3. Bootstrap detects (or does not detect) an install of FlintMC
   4.1 FlintMC is not detected -> Bootstrap sets up a FlintMC install
   4.2 FlintMC is dectected -> Bootstrap verifies the integrity of the install and possibly checks for updates
4. Bootstrap performs additional checks regarding installed mod compatibility
5. Boostrap loads FlintMC launcher plugin and injects it using the provided mechanism
6. The launcher finishes the first loading cycle and begins the second one
7. The launch process continues as known