runDialog: Explicitly set horizontal alignment

When set to fill, the label will always end up left-aligned, which
is only correct in LTR locales. Set the alignment explicitly to
work in both RTL and LTR locales.

https://bugzilla.gnome.org/show_bug.cgi?id=712579
js: Use SOURCE_CONTINUE/SOURCE_REMOVE constants in source functions

With support for boolean constants in g-i, we can finally use the
more readable constants instead of true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567
js: Use EVENT_PROPAGATE/EVENT_STOP constants in event handlers

Just as SOURCE_CONTINUE/SOURCE_REMOVE in source functions, these
constants increase code clarity over plain true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567