Added contextual menus in filter line box and cells for assisting in using
filters and for discovering existing filters.
The menu in the filter line adds "What's This" option and helper options
that add the operator and a selected placeholder (?) for easy editing.
Special options for NULL and empty strings are also added. Those work
directly.
The menu in the cells works as the current "Use as Filter" but for
different operators. In this case, only the range operator is incomplete.
Currently, not all the operators make sense for strings because we take
them as LIKE filters. At least the Not Equal filter (<>) should work, but
the other would also make sense. This will be addressed in future commits.
When changing between the filter line edits in the Browse Data tab, make
sure to only re-run the SQL query if the value of the filter line edits
have actually changed.
See issue #1187.
When changing a filter value delay the application of the new value on
the table for a couple of milliseconds. This makes the filter process
much smoother for fast typing users working on large tables.
See issue #415.
