Better forcefield control dialog

The size configuration is no longer cleared when exiting the dialog with
<esc>.  The enable/disable toggle button now indicates the current state.
The name of the toggle button now varies according to state, so that
pressing the button multiple times in one state (which can arise due
to lag making the user unsure about whether the first press registered)
only makes the state change that the user requested, rather than toggling
repeatedly.