Add View::on_remove, better ExprTK coercion

[texodus]

• Fixes Table 'remove' operation does not reflect jupyter widget data #1597 and its duplicate removing rows on backend table does not replicate to client table #2293 via a "new" View::on_remove callback registration which ticks Arrow encoded IDs evicted from indexed Tables. "New" because this API existed in v2.x, but did not trigger in the commonly used (but poorly performing) use case of calling Table::clear and Table::replace, followed by a Table::update with the same slightly modified dataset. This "new" version handles both cases correctly, and uses Arrow encoding to lessen the performance impact - however, nothing will ultimately change that this is the slowest way to use Perspective, and incremental changes via Table::update and Table::remove should be preferred where possible.
• Fixes Comparison operators do not work with data types other than float in "Expressions". #1614 by introducing complete numeric type coercion to Perspective's ExprTK validation pass, as well as a few other small type fixes with operators, nulls and argument types. Casting numeric types to compare them (and having them silently fail otherwise) is no longer an issue and most types will coerce as you expect when compared, regardless of underlying type.
• Fixes unreported resize issue in perspective-viewer when text outside the component is selected and a drag/drop operation is attempted.