Rename fields in the Symbol Fields Table window

Allow some changes of user fields when into the "Bulk-edit fields of all symbols in schematic" table-windows:

1. Delete [already implemented];
2. Rename;
3. Change the presentation order [already implemented, drag header row];
4. Add others.

To achieve this, add the follow buttons and a right-button context menu when clicking in an user field. The "system" fields (datasheet, value, reference) should not allow those changes, keep the buttons blocked and not display the context menu.

Application: Eeschema
Version: 5.99.0-unknown-c3175b4~86~ubuntu16.04.1, release build