Implement the ability to customise shell arguments via dconf.

Added a shell-args key to implement the functionality outlined in
p-e-w/finalterm#100. To make your shell a login shell, simply set the
value of aforementioned key to the following:

    ['-l']
Update about dialog credits; small code layout adjustment
Fix #71

The problem was a strange condition where the layout manager
sometimes first allocates a large space (say, 20 x 20 characters in size)
for the line container, which triggers a (delayed) resize cycle,
and then immediately afterwards allocates a smaller, often invalid
(20 x 0) space, aborting a legitimate resize.
Added ```ls``` option to directory menu. (Fix #109)

Changed ```ls``` text from "View contents" to "List contents" as per the ls man page, and passed ```-lh``` to ```ls```.