Correctly check for and use Qt4's qmake

Instead of just using qmake, check if either qmake or qmake or qmake-qt4
exists and uses version 4 of Qt. This affects both the dependency check
and the actual compilation of Dwarf Therapist.

This also generally fixes the dependency check. Previously, it actually
tested for Mercurial a second time.
Query QT_VERSION instead of using qmake -v
Separate qmake detection from dependency check

find_qmake_qt4 is now a separate function called by
build_dwarf_therapist and check_dependencies.
Revbump to 0.2.4, fixing Qt4 detection.
Add ptrace_scope check to dependency checking.
Fix annoying issue where soundsense expected gamelog in ../ by editing configuration.xml on the fly.
Add rudimentary (aka same as save_backup) backup and restore of soundsense audio packs.
Rework most calls to exit_with_error.
Add complete df folder backup with selective restore based on what issue(s) occurred.
Fix a few minor cleanup issues.
Fix issue where an upgrade from a folder that didn't contain a DF install would crash. Now it just errors nicely.
Add g++ and gcc checks as part of resolving https://github.com/andrewd18/df-lnp-installer/issues/20
Update steps for installation. Add Upgrade steps. Revbump Falconne plugin.
Add common issues section to the readme.
Fix link.
Add --skip-sha option.
Document --skip-sha argument.
Update ptrace warning message to point to the wiki.
Downloading files is slightly more modular (yay!) and errors when the download fails rather than exploding at the checksum or install phase.
Change Dwarf Therapist repo download steps to retroactively fix 0.2.0 repository fail (in the slim chance there are people with the wrong repo).
Add a DF LNP Logo and install it to the install directory.
Build a .desktop file on the fly and install it in the user's home folder.