Forgot a gitignore

See merge request STJr/SRB2!362

Installer followup

See merge request STJr/SRB2!361

* Check if administrator rights are required
* Add INSTALL INSTRUCTIONS text file to root install folder in case the installer fails
* Add failsafes to uninstaller file deletion: check if name is a folder, and check for illegal chars
* Fixed parentheses breaking the installer by instead making exclamation marks do the breaking. Thanks Windows!

Add new Windows installer scripts

See merge request STJr/SRB2!356

Update debian packaging to work with Launchpad PPA

See merge request STJr/SRB2!350

Save sound toggles to config

See merge request STJr/SRB2!348

This reverts commit fd12e856.

Hide OpenGL loading screen

See merge request STJr/SRB2!349

FOF wall slope skewing backport

See merge request STJr/SRB2!341