Make prompt dialogs explicitly modal

dialogs.c:show_prompt() uses dialog_run which acts as modal on GTK
based window systems because it uses a recursive mainloop, but
appears to not do so on KDE or Unity window systems. See [Bug #976](https://sourceforge.net/p/geany/bugs/976/).
Explicitly setting the modal flag as well may be honoured by those
systems.

This PR is to let the OP test it first on KDE and Unity.