Fix user template icons not displaying on Windows

Absolute Windows paths were not being checked for, causing
user template icons to not load. QFile::exists is a simple
cross platform method of checking for an absolute path.

This also fixes custom icons for user templates.

BUG: 395894

libs/ui/KisTemplate.cpp

@@ -50,7 +50,7 @@ const QPixmap &KisTemplate::loadPicture()
 
     m_cached = true;
 
-    if (m_picture[ 0 ] == '/') {
+    if (QFile::exists(m_picture)) {
         QImage img(m_picture);
         if (img.isNull()) {
             dbgKrita << "Couldn't find icon " << m_picture;