[app] Add support for enabling QML debugging

Introduce new environment variable to enable and configure
QML debugging when starting Meshroom GUI app.

Introduce a new EnvVar class to centralize the management of
environment variables across Meshroom.

meshroom/env.py

@@ -0,0 +1,53 @@
+"""
+Meshroom environment variable management.
+"""
+
+__all__ = [
+    "EnvVar",
+]
+
+import os
+from dataclasses import dataclass
+from enum import Enum
+import sys
+from typing import Any, Type
+
+
+@dataclass
+class VarDefinition:
+    """Environment variable definition."""
+
+    # The type to cast the value to.
+    valueType: Type
+    # Default value if the variable is not set in the environment.
+    default: str
+    # Description of the purpose of the variable.
+    description: str = ""
+
+    def __str__(self) -> str:
+        return f"{self.description} ({self.valueType.__name__}, default: '{self.default}')"
+
+
+class EnvVar(Enum):
+    """Meshroom environment variables catalog."""
+
+    # UI - Debug
+    MESHROOM_QML_DEBUG = VarDefinition(bool, "False", "Enable QML debugging")
+    MESHROOM_QML_DEBUG_PARAMS = VarDefinition(
+        str, "port:3768", "QML debugging params as expected by -qmljsdebugger"
+    )
+
+    @staticmethod
+    def get(envVar: "EnvVar") -> Any:
+        """Get the value of `envVar`, cast to the variable type."""
+        value = os.environ.get(envVar.name, envVar.value.default)
+        return EnvVar._cast(value, envVar.value.valueType)
+
+    @staticmethod
+    def _cast(value: str, valueType: Type) -> Any:
+        if valueType is str:
+            return value
+        elif valueType is bool:
+            return value.lower() in {"true", "1", "on"}
+        return valueType(value)
+