Use the build tools written in C

The build tools in C are far from finished, but it's better to use them
as soon as possible, so it will be better tested.

This commit also renames Latexila -> LatexilaApp (in Vala), because
there is a conflict with the Latexila namespace used by the liblatexila.
MainWindowBuildTools: connect to the loaded signal
BuildToolsDefault: fix GVariant handling

The GVariant here has a floating reference taken by GSettings, so no
need to unref the GVariant.
Fix the saving of the personal build tools

Hold the application alive when the personal build tools are saved.
And do the save (asynchronously) when the BuildToolsPreferences dialog
is closed.
latexila_utils_replace_home_dir_with_tilde() in C

The function comes from gedit.
BuildView: replace home dir with tilde in the path
More utilities functions

With unit tests and documentation of course.
BuildView: add icons
Build tools: implement the run operations (not finished)

And adds a str_replace() utils function.