Move reloading of theme outside of the action thread

Reloading the GUI while the GUI is running and rendering is very
dangerous as we may be deleting GUI resources before or while we
are trying to render those same items. This change will allow us
to safely load new resources and delete the old ones.

Fixed a double free situation that was causing a crash on every
other theme reload.

Clear parsed xml documents when we are done loading the theme
instead of keeping them in memory for the life of the theme to
help save a little memory.

Store starting page so that if we request a reload, we can enter
the theme at the same starting point, which may come in handy if
we allow for language selection on alternate starting pages such
as the decrypt prompt or the system read only prompt.

Change-Id: I45a7e3fb3daeefac56d70f8d4936938eb1244b99