Save the last opened Amp & IR

[prachurjyadutta]

When we close nam in the standalone mode, we have to re-select the amp and IR models once we reopen it.

We would like if it stored the last used amp and the IR model automatically reloads once we open nam.

We could also have a default amp and IR model, i.e. you set it as default and it opens that combo when you open nam,

[sdatkinson]

Needs #428