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.

CMAKE_SYSTEM_ARCHICTETURE always returns the *host* not target arch
when using the MSVC generators. (CMake bugs 15170 and 14342.)

citra-qt: Separate UI layout state in a separate section of the config

citra-qt: Trim recently used files list to size when insterting new item

Closes #1113

Even though they weren't visible in the UI, old entries would never be
removed from the list and would be stored in the config file across
sessions.

citra-qt: Use monospace font on Disassembler and ARM Registers

Shader: Use constants and proper type casts

Shader Debugger Improvements

This avoid some annoying focus stealing in some situations, and looks
nicer in general.