Option for generating pronounceable passwords with pwgen

This commit adds an option in the settings dialog to generate passwords with the external program `pwgen` instead of the build-in algorithm.  This option is hidden if `pwgen` is not installed.  The default is disabled.

Pwgen passwords may be preferable because they are "pronounceable" and easier to copy by hand into things like phones.  As an example, pwgen passwords look like:
```
equeighiRoP6
phau5Cahs9ea
jaa8EiXie8Ai
ORecoh1ni4De
```
Loxodo (default) passwords look like:
```
Vb_iCBzGIvh0
_C5SfeMqAwda
t5Fsn2cVVSbK
1sbKNvakT_fQ
```