Fix: Key event not being converted properly if there's a modifier

Currently, if we map a key to LMB and we try to press for example shift
key - which adds a modifier on top of the mapped key to LMB, then if the
user stops pressing the key which is mapped to LMB - Aseprite won't
register it as kMouseUpMessage event, and if we try to draw a rectangle
with aspect ratio (by pressing shift for example), and then we stop
holding the mapped key to LMB, drawing will not stop.

This is because Accelerator class doesn't recognize the mapped key with
the shift modifier the same as if it was pressed without the shift modifier.
For example, if the key mapped to LMB is 'V', then if we use Rectangle
Tool, and then press Shift + V and then stop holding V - it should stop drawing
current rectangle, since we stopped holding the key mapped to LMB. But
this doesn't happen, because of the key event coming with a modifier.
Accelerator class sees the event as Shift+V, not just 'V' as the key
which is mapped to LMB, thus it is not converting it to "MouseUp" event in
doc_view.cpp:onProcessMessage method, and because of that drawing is not
stopping.

The solution for that is to simply add a second check to
Accelerator::isPressed method, it simply compares currently checked object
to the scancode without the modifier, allowing for the key event to be
converted.

src/ui/accelerator.cpp

@@ -330,6 +330,7 @@ std::string Accelerator::toString() const
 bool Accelerator::isPressed(KeyModifiers modifiers, KeyScancode scancode, int unicodeChar) const
 {
   return ((scancode && *this == Accelerator(modifiers, scancode, 0)) ||
+          (scancode && *this == Accelerator(kKeyNoneModifier, scancode, 0)) ||
           (unicodeChar && *this == Accelerator(modifiers, kKeyNil, unicodeChar)));
 }