Redesign "Incomplete folder" feature

Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).

src/base/utils/fs.cpp

@@ -72,9 +72,14 @@ QString Utils::Fs::toUniformPath(const QString &path)
     return QDir::fromNativeSeparators(path);
 }
 
-/**
- * Returns the file extension part of a file name.
- */
+QString Utils::Fs::resolvePath(const QString &relativePath, const QString &basePath)
+{
+    Q_ASSERT(QDir::isRelativePath(relativePath));
+    Q_ASSERT(QDir::isAbsolutePath(basePath));
+
+    return (relativePath.isEmpty() ? basePath : QDir(basePath).absoluteFilePath(relativePath));
+}
+
 QString Utils::Fs::fileExtension(const QString &filename)
 {
     const QString name = filename.endsWith(QB_EXT)

src/base/utils/fs.h

@@ -42,6 +42,7 @@ namespace Utils::Fs
      * with the OS being run.
      */
     QString toNativePath(const QString &path);
+
     /**
      * Converts a path to a string suitable for processing.
      * This function makes sure the directory separator used is independent
@@ -50,7 +51,16 @@ namespace Utils::Fs
      */
     QString toUniformPath(const QString &path);
 
+    /**
+     * If `path is relative then resolves it against `basePath`, otherwise returns the `path` itself
+     */
+    QString resolvePath(const QString &relativePath, const QString &basePath);
+
+    /**
+     * Returns the file extension part of a file name.
+     */
     QString fileExtension(const QString &filename);
+
     QString fileName(const QString &filePath);
     QString folderName(const QString &filePath);
     qint64 computePathSize(const QString &path);