Improve PGSQL TIMESTAMP support

There are two things the current data editor (build 4903) doesn't support properly for TIMESTAMP columns when inserting new rows:

1. It is not possible to revert the value to 'empty' (default value) after setting it to NULL, for example. This *is* possible however for the primary key.

2. The TIMESTAMP column supports microsecond precision and timezones but the "Insert Value" submenu only offers precision to a second. When editing a TIMESTAMP value, the microseconds and timezone disappear.

Oh, the second issue is probably solvable. I have added support for microseconds in MySQL some time ago. So I probably just have to allow the PostgreSQL timestamp type in some block of code.

I just created such a table and was able to edit the microseconds part of the "y" column:

CREATE TABLE "test2" (
"id" INTEGER NULL DEFAULT NULL,
"v" VARCHAR(50) NULL DEFAULT NULL,
"z" TIMESTAMP NULL,
"y" TIMESTAMP(3) NULL
)

But I just see you meant the "Insert value" submenu in the grid, right? Well, there are just some generic date/time values. I guess you never need these to really insert usefull values, do you?

Part of #2 is the submenu, and I could understand how that would not really be useful.