Allow choosing <> over "" for qdbusxml2cpp -i

qdbusxml2cpp's -i option always uses "" for the includes. Sometimes, one would like it to use <> for them, because some compilers may use a different search strategy for <> than for "" (e.g., including or excluding files in the same directory).

Acceptance criteria:

1. qdbusxml2cpp has a new command line option (--global-include/-I, naming subject to bikeshedding), which will include the given argument using <> instead of -i's ""
2. Use of the new switch is rolled out in at least qtconnectivity