[RFC] Allocation optimization for translation cases

[Therzok]

Currently, GettextCatalog.GetString has the following behaviour:

1. Take the parameter string and marshal it to native
2. Take the translated return value and marshal it to managed

When assigning a string to a widget property, the following happens:

1. The managed string is marshalled to native handle
2. The string is then strdup'd in native

The usual pattern is:

1. call GettextCatalog.GetString("value")
2. a. Assign the string to widget property
3. b. Format the string and then assign it to widget property.

That means we have a total of:

• 3 native allocations and 1 managed allocations for the simple translatable string
• 3 native allocations and 2+N managed allocations for the formattable translatable string (where N is the number of format arguments)

The 3 native allocations are: marshal to native for gettext, marshal to native for setting the widget property and 1 native allocation for strdup'ing the string.

The managed allocations are: native to managed for gettext, and in the latter case, string.format

Problem:
❗️ We allocate too many strings and marshal them unnecessarily.

Pre-requisites:
❗️ Gtk# and/or Xamarin.Mac need to support setting a string handle directly, rather than doing the marshalling for us.

Whilst we cannot do anything for the formatting case, as we need a managed string to format, the simple assignment case can be reduced to just:

widget.LabelHandle = GettextCatalog.GetStringHandle ("value")

The result:
1 managed to native allocation -> gettext
1 native allocation -> strdup when handle is set in native