[guilib] rename GUI_MSG_(DE)SELECTED -> GUI_MSG_SET_SELECTED to make it obvious what it does
[guilib] adds GUI_MSG_IS_SELECTED message and use this instead of querying radiobuttons directly
[guilib] use GUI_MSG_ITEM_SELECTED for getting the selected value of a spinner rather than querying it directly
[guilib] adds GUI_MSG_SET_LABELS for setting labels on a spinner, and use it in preference to direct setting on the control
[cosmetic] drop unused macro
[guilib] adds GUI_MSG_SET|GET_FILENAME to get and set the filename from an image control instead of querying directly
[guilib] use various GUI_MSG_* routines to save querying specific control types
[guilib] use SET_CONTROL_LABEL2 and GUI_MSG_SET_TYPE for setting/fetching info from edit controls rather than direct access
[guilib] use GUI_MSG_ITEM_SELECT to set the progress on progressbars and sliders
[gui] convert a bunch of GUI_MSG_RESET|ADD|SELECT_LABEL to use SET_CONTROL_LABELS
[cleanup] convert a few g_windowManager.SendMessage()'s to OnMessage()
Merge pull request #4739 from jmarshallnz/control_access_cleanup

Control access cleanup