Support for explicitly specified row heights

[telamonian]

Feature Request

Description of Problem:

As per the conversation here, one interesting feature that @lumino/datagrid has that regular-table is currently lacking is support for explicitly specifying row height.

For at least some set of possible "fat" rows (eg double-height, img with set height, etc), it seems reasonable that the <regular-table> author would know in advance what the "height anomaly" would be for a given row. It would be neat if we could support this use case.

Potential Solutions:

The ideal solution would allow for:

• correct calculation of virtual scroll position, such that there is parity in behavior, thumb size, etc between the virtual scrollbar of a <regular-table> and the scrollbar of an eagerly rendered vanilla <table>
• preservation of current regular-table virtual rendering, in which only the for-real visible rows get rendered

As implemented in datagrid, the idea is that you can deal with the various complications introduced by variable row height in O(n) time, where n is the count of rows with custom heights that are affected by a given operation (eg see here). For rendering, n would only be the count of currently visible non-default-height rows.

Non-solutions:

This feature is distinct from the bug discussed in #96, whereby if any row is larger than the default 19px it causes the breakage of the calculation of number of rows to render. The solution for #96 just has to prevent the complete breakage of an affected <regular-table> (eg it doesn't have to completely fix the virtual scroll behavior, etc). Also hopefully there will be a reasonable solution for #96 that won't require explicitly known/specified heights

[timkpaine]

#191