Fix pinentry configuration for GUI

[dashohoxha]

On cmd_init we set the configuration of gpg-agent to pinentry-program /usr/bin/pinentry-tty. Maybe the initialization for the GUI case should be a bit different (for example if we run egpg gui init instead of egpg init).
In this case the package pinentry-gtk2 or pinentry-gnome3 becomes a dependency.
Or maybe we can add the configuration of pinentry on Setting, so that the user can choose which one to use.

[diveshuttam]

Few options to implement this:

1. use a custom script for pinentry which changes pinentry to the ones we require based on an environmental variable. refer this
2. use 2 different gpg-agent.conf files for gui and cli and reload gpg-agent with the specific files
3. use something like the method being used by autopin in tests: change the pinentry program line in the gpgagent-conf file (using sed) and reload agent.

PS In all these options we can offer user to specify pinentry-programs for gui and cli in config.sh