Simplify code for setting a box in the GUI to be a password box

Just a minor refactor.
Make MS Access table_exists more consistent with other engines

Queries the table names using the ODBC .tables() method
and only does so once.

Use of the .tables() method instead of a query is necessary
because Access doesn't provide permissions to query the relevant
system table by default.

This should make MS Access robust to the kinds of race condition
errors identified fixed in eb702c6baf116164f265deca9303aa59e4187e36
for MySQL.
Make Access table checking work with pypymsql instead of pymysql

We are using pypymysql which returns a slightly different object
when calling the tables() method.
Save full path of database when saving connections involving files

This seems like the better behavior if a file with a relative
path is entered when making a connection.
Force connection wizard to be large enough for 'choose file' button

Because (probably) app.ConnectPage.Draw contains different fields
depending on the engine, wx is not properly accomodating the
varying sizes of this page that might result. In cases where there
is a file attribute (sqlite, MS Access) the wizard is too small
on some systems and the button is cut off.

This is a hack that temporarily addresses this problem by adding
a large empty text box to the bottom of the first page of the
wizard. This probably should be fixed properly when time allows.
Merge pull request #155 from ethanwhite/msaccess-table-exists

Make MS Access table_exists more like other engines
Merge pull request #156 from ethanwhite/save-full-path

Save full path of database when saving connections involving files
Merge pull request #157 from ethanwhite/dbconnect-size

Force connection wizard to be large enough for 'choose file' button