fix macos big sur issue

Revert "remove QT_X11_NO_MITSHM argument". Also make fixes to windows…

… compilation.

This reverts commit 1c3ff12.

changed default ubuntu download release to 20.04

added script for signing code, and signed frameworks

added missing file 7z.so

change default ubuntu version to 18.04

change default ubuntu version to 18.04

there is no i386 iso for ubuntu 17.10

fixed mkbootable not unmounting disks correctly

fix incorrect mkbootable command