[GSoC] Support editing resultsets from tables with OIDs

Hi,

Please find a patch attached to add supporting of resultsets from tables with OIDs as updatable resultsets (with updated test cases and docs) - this is a part of a GSoC project. Please review !

This patch also includes a small refactor of the previous patch and a small bug fix that occurred when the query resultset had no rows but was updatable (row additions were not permitted, although they should be).

On a different note, one of the feature tests fails (on master), probably due to a small typo. In query_tool_tests.py this function call:

self.page.find_by_xpath(
    '//div[contains(@class, "sql-editor-message") and '
    'contains(string(), "CREATE TABLE")]'
)

on lines 371-374 produces an exception that the element cannot be found, this is probably due to the fact that the message returned by the query executed before this call does not contain the string "CREATE TABLE". Just letting you know about this.

Thanks and regards!

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