api: check for window focus when accepting input (#317)

## Link to GitHub Issue, if one exists
Fixes #139 

## Description of change
Add an option for detecting window focus when accepting input.

The new default will be to have the following:
 * naive bindings will check for window focus
* rawinput binds are not affected, will continue to be accepted
regardless of focus

Other options (always / never choices) can be picked by the user to
tweak the behavior if needed.

Clean up usage of `0xff` constant to represent invalid naive vkey.

## Testing
Still in progress, especially in regards to performance.

On a i7-9700 system, 10000 calls to `superexit::has_focus()` takes
250-350 microseconds, so this should not be a big deal at all.

src/spice2x/cfg/button.cpp

@@ -284,7 +284,7 @@ std::string Button::getDisplayString(rawinput::RawInputManager* manager) {
     std::string vKeyString = fmt::format("{:#x}", vKey);
 
     // device must be existing
-    if (this->device_identifier.empty() && vKey == 0xFF) {
+    if (this->device_identifier.empty() && vKey == INVALID_VKEY) {
         return "";
     }