Replace application icons with Material Design icons.

Use the following to run KeePassXC with the icons from the
source code, ignoring the operating system's Qt icon theme:

```
KEEPASSXC_IGNORE_ICON_THEME=1 keepassxc
```

The patch further adds a script `makeicons.sh` that re-creates KeePassXC
icons from the Material Design icon set and can be used for easily
updating icons in the future. Instructions are in the script.

Fixes #475

src/core/FilePath.cpp

@@ -143,7 +143,7 @@ QIcon FilePath::icon(const QString& category, const QString& name, bool fromThem
         return icon;
     }
 
-    if (fromTheme) {
+    if (fromTheme && !getenv("KEEPASSXC_IGNORE_ICON_THEME")) {
         icon = QIcon::fromTheme(name);
     }