Add basic settings framework

This settingsframework uses mlite5 on SailfishOS and other platforms
with the mlite library present, because I believe DConf is neat. For
platforms that do not have DConf and mlite present, it falls back to a
quickly put together implementation that uses QSettings as a backend.

Implementing an settings item is simply done by subclassing the
Jellyfin::QObjectSettingsWrapper.

cmake/Findmlite5.cmake

@@ -0,0 +1,39 @@
+# - Try to find mlite5
+# Once done this will define
+#  mlite5_FOUND - System has mlite5
+#  mlite5_INCLUDE_DIRS - The mlite5 include directories
+#  mlite5_LIBRARIES - The libraries needed to use mlite5
+#  mlite5_DEFINITIONS - Compiler switches required for using mlite5
+
+find_package(PkgConfig)
+pkg_check_modules(PC_mlite5 QUIET mlite5)
+set(mlite5_DEFINITIONS ${PC_mlite5_CFLAGS_OTHER})
+
+find_path(mlite5_INCLUDE_DIR mlite-global.h
+    HINTS ${PC_mlite5_INCLUDEDIR} ${PC_mlite5_INCLUDE_DIRS}
+    PATH_SUFFIXES sailfishapp )
+
+find_library(mlite5_LIBRARY NAMES mlite5 mlite libmlite libmlite5
+    HINTS ${PC_mlite5_LIBDIR} ${PC_mlite5_LIBRARY_DIRS} )
+
+include(FindPackageHandleStandardArgs)
+# handle the QUIETLY and REQUIRED arguments and set LIBmlite5_FOUND to TRUE
+# if all listed variables are TRUE
+set(mlite5_VERSION ${PC_mlite5_VERSION})
+find_package_handle_standard_args(mlite5
+    FOUND_VAR mlite5_FOUND
+    REQUIRED_VARS
+        mlite5_LIBRARY
+        mlite5_INCLUDE_DIR
+    VERSION_VAR mlite5_VERSION)
+
+if(mlite5_FOUND AND NOT TARGET Foo::Foo)
+  add_library(mlite5::mlite5 UNKNOWN IMPORTED)
+  set_target_properties(mlite5::mlite5 PROPERTIES
+    IMPORTED_LOCATION "${mlite5_LIBRARY}"
+    INTERFACE_COMPILE_OPTIONS "${PC_mlite5_CFLAGS_OTHER}"
+    INTERFACE_INCLUDE_DIRECTORIES "${mlite5_INCLUDE_DIR}"
+  )
+endif()
+
+mark_as_advanced(mlite5_INCLUDE_DIR mlite5_LIBRARY )