Automatic IME configuration

[mominul]

OBK v3.0.0 will support two engine backends: iBus and Fcitx5. For not confusing the user and having a seamless integration, OBK will have both IMEs in the package.

For the popular distributions and where we can identify the best IME for the Desktop Environment, we'll automatically configure the DE so no manual configuration will be needed.

For Gnome Desktop environment, we'll configure for iBus
For non-Gnome DE, we'll configure for Fcitx5

[mominul]

I have already implemented automatic configuration for Gnome on auto_config branch

[mominul]

Implemented by #459