arxcrashreporter: Add support for Qt 5

Along with two new CMake variables:
- USE_QT5: Use Qt 5 if available
- USE_QT4: USe Qt 4 if available

If both USE_QT5 and USE_QT4 are enabled we will try to use Qt 5 if and
then fall back to Qt 4.

Both are ignored if BUILD_CRASHREPORTER is disabled.

Implements: task #500
Fix footnotes in README.md and OPTIONS.md
Merge remote-tracking branch 'upstream/master'