[2.7.1] Feature: Modloader UI for selecting mechs in the custom mech screen

[KnightMiner]

With the update to 1.2.75, we now have space for up to 18 mechs in each spot in the custom squad, it might be nice to be able to configure which mechs show up there.

I believe all mechs selected in squads are forced to show, but we could add addition mechs afterwards. Even beyond the initial 18, having them available in random squads would be nice.

[KnightMiner]

Full design:

Modder functions

• modApi:addMech: add a new mech to the custom squad screen, shows regardless of selected squads. feature-add-individual-mech is working on this.
• modApi:addSquad will gain a new option for "extra mechs" which will conditionally show in custom squad screen as long as the squad is enabled. This replaces the hack of adding the mechs after the 3rd mech in the array (internally its the same hack, but it looks like an API)

UIs

Mech selector is a new UI that allows selecting which extra mechs show in custom squads. By default, all mechs added through modApi:addMech are checked, and all squad mechs from modApi:addSquad are unchecked but available.

Any mechs in the selected squads may still be checked, but will be filtered out in the hangar UI.