Convert stored pixbufs to gtkmm
themer: Put all GUI code into a class
themer: Add copyright header to main.cc
themer: Move ThemerWindow class to a separate pair of files
themer: define LAST_COMPAT_VER in a separate header
themer: Fix calls to ThemerWindow::refresh_theme_list()
emerald: Convert GHashTable to std::unordered_map
Partially revert commit a0299affb3

Some of the commented-out code was code that was not yet converted to
gtkmm
themer: Add ThemerWindow::searchbox_ to reduce passing it by parameters
Use C++11 lambdas instead of bind and mem_fn in signal handlers
Refactor setting tables so that no global state is used
Clear obsolete TODO
libengine: Fix begin(), end() ADL for directory iterators
themer: Minor refactoring of callbacks
themer: Improve variable naming
themer: Extract theme list handling into a separate class