Add option to automatically remove .torrent files upon adding

Some browsers do not download files, intended for immediate opening,
into a temporary directory, and thus a regular download directories
accumulate those unneeded files.

The option allows qBittorrent to clean after itself and delete those
files whether they were succesfully added or not (user-selectable
policy).

src/base/base.pri

@@ -51,6 +51,7 @@ HEADERS += \
     $$PWD/utils/misc.h \
     $$PWD/utils/string.h \
     $$PWD/unicodestrings.h \
+    $$PWD/torrentfileguard.h \
     $$PWD/torrentfilter.h \
     $$PWD/scanfoldersmodel.h \
     $$PWD/searchengine.h
@@ -103,6 +104,7 @@ SOURCES += \
     $$PWD/utils/gzip.cpp \
     $$PWD/utils/misc.cpp \
     $$PWD/utils/string.cpp \
+    $$PWD/torrentfileguard.cpp \
     $$PWD/torrentfilter.cpp \
     $$PWD/scanfoldersmodel.cpp \
     $$PWD/searchengine.cpp