Table row selection

**Is your feature request related to a problem? Please describe.**

I'd like to write an egui application with a typical master-detail interface: On the left-hand side, there is a table of things; on the right-hand side, the thing(s) currently selected in the table are displayed with more detail. Think your typical e-mail client.

It is my understanding that the table component currently doesn't support selecting rows.

**Describe the solution you'd like**

Ideally, I'd like the following features:

* Choice between single-row selection (at most one row can be selected at any time) and multi-row selection (any number of rows can be selected at a time)
* Clicking on any row selects this row, unselecting any other rows
* Pressing the up or down cursor keys selects the single row above or below the current selection
* In multi-row mode, the following additional interactions are possible:
* Ctrl-clicking allows adding individual rows to the selection
* Shift-clicking allows selecting a range of rows
* Ctrl+A selects all rows