May fix additional texture caching issues. (Though mostly in homebrew, I
haven't seen any commercial software use this to flush anything but
command lists.)

Replace 0 literals with nullptr where applicable

synchronized_wrapper: Add missing return in SynchronizedRef move assignment operator

General: Replace NULL and '0' usages with nullptr where applicable

graphics_breakpoints_p: Add missing override specifier

citra_qt: Reorganize headers

video_core: Reorganize headers

common: Get rid of debug_interface.h

common: Get rid of a cast in swap.h

This is technically unused. Also removes TMemChecks because it relies on this.
Whenever memory breakpoints are implemented for real, it should be designed to
match the codebase debugging mechanisms.

video_core: Remove unnecessary includes from headers

memory: Get rid of pointer casts

CMake: Make all cache options appear even in case of errors

gl_rasterizer: Replace push_back calls with emplace_back in AddTriangle

renderer_opengl: Remove unimplemented function declaration

General: Fix up doxygen comments

video_core: Remove unused variables

y2r: Give local variables an initial value

disk_archive: Remove unimplemented constructor declarations

Keeps compilers/static analyzers quiet.

The `option` commands have been moved to the top of the file, so that
the relevant options are registered in the CMake cache even if one of
the required libraries is not found. This solves an ergonomic problem
when using bundled libraries where you have to first download GLFW
before being able to select the option to also download Qt.

CMake: Add option to download Qt binaries

Using this variable is problematic is the user has several versions of
Qt installed on their system. There is no way to know ahead of time if
the Qt version pointed to by QTDIR matches the toolchain that is being
targeted.

The Qt installation path can still be easily specified if it's not found
by CMake by setting the Qt5_DIR cache variable after the initial
configuration run, so this shouldn't present an usability issue.