Search in selection

[gitterspec]

Describe the problem to be solved

Search can return too many hits when there's a lot of columns. This was also mentioned here https://github.com/gristlabs/grist-core/issues/105 among other idea, but I think it deserves its own Issue.

Describe the solution you would like

Mirror Excel's behavior, which is pretty common in other apps too. When more than one cell is selected, search will default to searching in the selection only.

For example, one can constrain the search to a specific column by selecting that column before searching. Useful for jumping to a specific record in mind when there's a lot of rows.