Fix: TextfileWindow called virtual methods before constructor completed. (#11889)

SetStringParameters() was called during widget tree init in the constructor.

Calls within a constructor cannot call the derived classes methods. This would result in invalid data being passed to the string system, which could then crash.

Codechange: Make all NWidgetPart arrays constexpr.

This ensures that the arrays are not created at runtime and prevents using non-constexpr values.

Codechange: Replace mishmash of types for widget index with WidgetID.

Indices were stored as int, but often passed around as uint/uint8_t and casts.

Now they should all use WidgetID.

Fix #11643: Empty area at top of survey preview. (#11647)

Empty area at the top of some textfile windows due to calling
SetDisplayedPlane() after calling FinishInitNested(), and/or changing
the displayed plane and not calling ReInit() after.

This was previously hidden by CheckForMissingGlyphs() reinitialising
all windows anyway.

Codechange: Add __FILE__/__LINE__ of WindowDesc to WindowDesc.

This is to allow unit-tests to produce more useful output.