The app hangs while viewing structure for large tables

While trying to view structure of a table with large number of columns ( 512+ ) the application hangs and takes a while to respond.

All that time I see a busy icon.

Works well for tables with smaller number of columns ( < 20 )

**Postico version:**
1.4.2

**macOS version:**
macOS Sierra v 10.12.4

**PostgreSQL version:**
PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.3906

Reproduce structure viewing with a table containing 512 or more columns, comparing it with a table under 20 columns. Use the reported Postico 1.4.2, macOS Sierra 10.12.4, and PostgreSQL environment as context. Done means the structure view responds without hanging or showing a prolonged busy state for large tables.

Written by the indexing model from the issue text.