Ability to set image as GDM background

Closes #40.

scripts/config.py

@@ -2,6 +2,7 @@ from tempfile import gettempdir
 
 # folder definitions
 temp_folder = f"{gettempdir()}/marble"
+gdm_folder = "gdm"
 gnome_folder = "gnome-shell"
 temp_gnome_folder = f"{temp_folder}/{gnome_folder}"
 tweaks_folder = "tweaks"

scripts/gdm.py

@@ -34,7 +34,7 @@ class GlobalTheme:
         self.theme_folder = theme_folder
         self.destination_folder = destination_folder
         self.destination_file = destination_file
-        self.temp_folder = os.path.join(temp_folder, "gdm")
+        self.temp_folder = temp_folder
 
         self.backup_file = f"{self.destination_file}.backup"
         self.backup_trigger = "\n/* Marble theme */\n"  # trigger to check if theme is installed

scripts/utils/is_photo.py

@@ -0,0 +1,21 @@
+image_extensions = [
+        'jpg', 'jpeg', 'png', 'gif', 'bmp',
+        'webp', 'tiff', 'tif', 'svg', 'heic', 'heif'
+    ]
+
+def is_photo(extension: str):
+    """
+    Check if extension is supported photo format
+    """
+
+    return extension.lower() in image_extensions
+
+class NotSupportedPhotoExtension(Exception):
+    """
+    Exception for not supported photo extension
+    """
+
+    def __init__(self, extension):
+        self.message = (f"Photo extension {extension} is not supported. "
+                        f"Supported formats: {", ".join(image_extensions)}.")
+        super().__init__(self.message)