Previously, AK::Function would accept _any_ callable type, and try to
call it when called, first with the given set of arguments, then with
zero arguments, and if all of those failed, it would simply not call the
function and **return a value-constructed Out type**.
This lead to many, many, many hard to debug situations when someone
forgot a `const` in their lambda argument types, and many cases of
people taking zero arguments in their lambdas to ignore them.
This commit reworks the Function interface to not include any such
surprising behaviour, if your function instance is not callable with
the declared argument set of the Function, it can simply not be
assigned to that Function instance, end of story.
Pull out the Label updating code into its own function.
Ideally, we should probably transform this code to use its own widget
rather than doing this all in-line.
I.e., having a `FontSettingWidget` that that ties the FontPicker, Label
and Button into its own little widget, so that we can stay extendible in
the main widget and reduce duplication some more!
Previously, the font was applied to the Labels but the name wasn't
updated on initial startup.
This meant that the Label's content was only correct in the default
state but not if the user changed the defaults.
