Add frege installations preference page

[yuxuanchiadm]

Add frege "Installations" preference page to global preference and "Frege Compiler" page project properties, So we can change frege version for projects to what we need. Not just built-in fregec FregeIDE contains. And also using library container to contain fregec dependency not just simple "Referenced Libraries"

[Ingo60]

You can replace the fregec.jar in the plugin's lib directory with a symbolic link that points to the place where you keep your latest fregec.jar.

TBH, I have no idea how to add another preference page. I'm also sceptical whether it is possible at all to change part of the plugin on-the-fly.