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.

If the first type of breakpoint to be hit wasn't "Vertex Loaded", the
input_vertex would contain garbage, which would be passed to the shader
interpreter and ocasionally cause crashes.

CMake: Add missing library required by Linux for compiling glad

DynCom: Converted all magic 0xE condition code checks to ConditionCode::AL

OpenGL: Use Sampler Objects to decouple sampler config from textures

Fixes #978

This stops (for some reason sporadic) crashes and OpenGL errors during
shutdown, when the OpenGL renderer tries to clean up objects from the
UI thread, which has no OpenGL context active.

Replace the previous OpenGL loader with a glad-generated 3.3 one

Add cam:u service function names to its function table