Add planets-char and aspects-char settings to GSettings schema
Add new settings planets-char and aspects-char to the preferences dialog

They are only effective upon chart redraw. A callback must be utilised
to automatically redraw the aspect table upon change.
Create planet and aspect symbols with respect to the (planets|aspects)-char settings
Refactor aspect table redrawing to a separate function

This is to later allow me to call when the certain GSettings values
((planets|aspects)-char) change.
Automatically apply settings change for planet/aspect icons as characters