Closes #326

When feh loses its controlling terminal at runtime, e.g. due to backgrounding /
disowning, it will no longer issue a warning on each terminal keystroke.

Silence -Wimplicit-fallthrough= warning

This allows finer control of which files are compiled and additionally
will silence some -Wpedantic warnings when "empty" files are compiled.

As there's no "default" window, it doesn't make much sense here...
Plus, it's buggy as hell in this mode

Also, restore the previous terminal mode on exit

Turns out that it is undefined behaviour to pass a value to isctype functions
which does not fit inside a char. Closes #312

(cf #309)

Closes #303

closes #302