Fix the properties dialog and a hidden memory corruption

The hidden memory corruption results from writing a pointer (8 bytes) to a gboolean variable (4 bytes) under certain conditions when the properties dialog is changed by the user. This might result in changing the low bits of a 64-bit floating-point variable following the gboolean in memory.

Dialog before this patch:

The fixed dialog (using exactly the same configuration file as the above image):

Closes: #4 (closed)

Mitigates: #8