This cleans up the external program execution in aescrypt-gui.

### Description ###

Fixes special character password entry, handles no password supplied gracefully, no longer requires repeated password entries for decrypting. Also fixed zenity window sizes.

### Testing ###

#### Versions ####

Tested on Ubuntu 13.10 Saucy using:
 - Unity/Zenity 3.8.0
 - KDialog 1.0 with Qt: 4.8.4, KDE 4.11.5.

#### Inputs ####

1. Expected success passwords:
    - quotinp'
    - slashinp\
    - !#$@*#"
2. Expected failure passwords (also fails in aescrypt, and aescrypt-gui displays the correct aescrypt error msg):
    - utf8©

#### Method ####

Used aescrypt-gui to encrypt and aescrypt to decrypt, and vice versa. Also tested mismatched password entry. Manually disabled zenity check to force KDE test.