Clean up command-line args and refactor QmlDownloader into 3 pieces

[slipher]

Combine 3 command-line args which are mutually exclusive and not intended for the end user into the --internalcommand argument.

Break qmldownloader.cpp into 3 parts. A GameLauncher takes the code for starting the game process; a SplashController for deciding what to do at the splash screen (whether an update is needed); and finally QmlDownloader keeps the downloading responsibilities.

TODO: test on Linux. So far I tested on Windows.

[illwieckz]

2.379 g++ -c -pipe -Werror -O2 -std=gnu++11 -Wall -Wextra -ffunction-sections -fdata-sections -D_REENTRANT -fPIC -DFLUID_LOCAL -DFLUID_STATIC -DGIT_VERSION=\"v0.2.0\" -DQUAZIP_BUILD -DQUAZIP_STATIC -DQT_DISABLE_DEPRECATED_BEFORE=0x050D00 -DQT_NO_DEBUG -DQT_QUICK_LIB -DQT_SVG_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_QMLMODELS_LIB -DQT_QML_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I/updater2 -I. -I/updater2/fluid/src/fluid -I/updater2/fluid/src/imports/core -I/updater2/fluid/src/imports/controls -I/updater2/quazip/quazip -I/qt/include -I/qt/include/QtQuick -I/qt/include/QtSvg -I/qt/include/QtWidgets -I/qt/include/QtGui -I/qt/include/QtQmlModels -I/qt/include/QtQml -I/qt/include/QtNetwork -I/qt/include/QtCore -I. -isystem /usr/include/libdrm -I/qt/mkspecs/linux-g++ -o updater2_plugin_import.o /build/updater2_plugin_import.cpp
2.495 /updater2/main.cpp:37:10: fatal error: startgame.h: No such file or directory
2.495  #include "startgame.h"
2.495           ^~~~~~~~~~~~~
2.495 compilation terminated.
2.507 make: *** [Makefile:4979: main.o] Error 1
2.508 make: *** Waiting for unfinished jobs....

[slipher]

I fixed the nonexistent include and tested it a bit on Linux.

[illwieckz]

I confirm it now builds and run on my side. Edit: on Linux.