PATCH: Limit 100 default in frmEditGrid.cpp

I propose a small patch to frmEditGrid.cpp that changes the default limit
form "No limit" to "100 rows".

Another option, more coherent in design to the rest of the program, can be
disabling automatic run and let the user configure the option before. I
think there are a small number of forms that are performing long operation
at the show time without the explicit user action.

Besides usability, the problem I found is a segfault happening when the
table is huge (mine is 14e6 records) and the frmEditGrid is closed before
the load is complete. In my configuration the crash is reproducible with
version 1.8.4 and trunk.

Cheers,
Matteo Bertini