Make inset-select-all select all cells only in tables

Make inset-select-all select all cells only in tables

inset-select-all has 3 levels
1. select current cell
2. select all cells
3. select inset from outside.

The second level makes sense for tables (text and math), but not for things like a math fraction.

Introduce a new method Inset::isTable() that allows to detect this case properly and skip level 2.

(cherry picked from commit 5de28b9ac6736c791146a1a3ebe7f4826c22a9b7)